ENH: support exprField specification for SemiImplicitSource

- this allows more flexibility when defining the location or intensity
  of sources.

  For example,

  {
      type            scalarSemiImplicitSource;
      volumeMode      specific;
      selectionMode   all;

      sources
      {
          tracer0
          {
              explicit
              {
                  type       exprField;

                  functions<scalar>
                  {
                      square
                      {
                          type square;
                          scale 0.0025;
                          level 0.0025;
                          frequency 10;
                      }
                  }

                  expression
                  #{
                      (hypot(pos().x() + 0.025, pos().y()) < 0.01)
                    ? fn:square(time())
                    : 0
                  #};
              }
          }
      }
  }

ENH: SemiImplicitSource: handle "sources" with explicit/implicit entries

- essentially the same as injectionRateSuSp with Su/Sp,
  but potentially clearer in purpose.

ENH: add Function1 good() method to define if function can be evaluated

- for example, provides a programmatic means of avoiding the 'none'
  function

tutorials/compressible/rhoPimpleFoam/RAS/mixerVessel2D/0.orig/tracer0

@@ -0,0 +1,47 @@
+/*--------------------------------*- C++ -*----------------------------------*\
+| =========                 |                                                 |
+| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
+|  \\    /   O peration     | Version:  v2206                                 |
+|   \\  /    A nd           | Website:  www.openfoam.com                      |
+|    \\/     M anipulation  |                                                 |
+\*---------------------------------------------------------------------------*/
+FoamFile
+{
+    version     2.0;
+    format      ascii;
+    class       volScalarField;
+    object      tracer0;
+}
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+dimensions      [0 0 0 0 0 0 0];
+
+internalField   uniform 0;
+
+boundaryField
+{
+    #includeEtc "caseDicts/setConstraintTypes"
+
+    rotor
+    {
+        type            zeroGradient;
+    }
+
+    stator
+    {
+        type            zeroGradient;
+    }
+
+    front
+    {
+        type            empty;
+    }
+
+    back
+    {
+        type            empty;
+    }
+}
+
+
+// ************************************************************************* //

tutorials/compressible/rhoPimpleFoam/RAS/mixerVessel2D/system/scalarTransport

@@ -33,9 +33,50 @@ tracer0
                 }
             }
 
-            injectionRateSuSp
+            sources
             {
-                tracer0     (0.01 0);
+                tracer0
+                {
+                    explicit constant 0.01;
+                }
+            }
+        }
+
+        source2
+        {
+            type            scalarSemiImplicitSource;
+
+            volumeMode      specific;
+
+            selectionMode   all;
+
+            sources
+            {
+                tracer0
+                {
+                    explicit
+                    {
+                        type       exprField;
+
+                        functions<scalar>
+                        {
+                            square
+                            {
+                                type square;
+                                scale 0.0025;
+                                level 0.0025;
+                                frequency 10;
+                            }
+                        }
+
+                        expression
+                        #{
+                            (hypot(pos().x() + 0.025, pos().y()) < 0.01)
+                          ? fn:square(time())
+                          : 0
+                        #};
+                    }
+                }
             }
         }
     }

tutorials/incompressible/pimpleFoam/laminar/planarPoiseuille/validation/WatersKing/createFields.H

@@ -49,10 +49,24 @@ IOdictionary fvOptions
         IOobject::NO_WRITE
     )
 );
-const dictionary& gradPDict =
-    fvOptions.subDict("momentumSource").subDict("injectionRateSuSp");
-const scalar K =
-    gradPDict.get<Tuple2<vector, scalar>>("U").first().x();
+
+scalar K(0);
+
+// Get x() component from U source
+{
+    const dictionary& momSource = fvOptions.subDict("momentumSource");
+
+    if (momSource.findDict("sources"))
+    {
+        K = momSource.subDict("sources")
+            .get<Tuple2<vector, scalar>>("U").first().x();
+    }
+    else
+    {
+        K = momSource.subDict("injectionRateSuSp")
+            .get<Tuple2<vector, scalar>>("U").first().x();
+    }
+}
 
 dictionary probes(IFstream(runTime.system()/"probes")());
 const point location = pointField(probes.lookup("probeLocations"))[0];