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

src/functionObjects/solvers/scalarTransport/scalarTransport.H

@@ -100,14 +100,12 @@ Usage
                     type            scalarSemiImplicitSource;
                     enabled         true;
 
-                    scalarSemiImplicitSourceCoeffs
+                    selectionMode   all;
+                    volumeMode      specific;
+
+                    sources
                     {
-                        selectionMode   all;
-                        volumeMode      specific;
-                        injectionRateSuSp
-                        {
-                            s           (1 0);
-                        }
+                        s           (1 0);
                     }
                 }
             }