ENH: additional uniform boundary conditions (#2703)

- provide uniformMixed conditions for finite-area and finite-volume.

  These are intended to replace the exprMixed condition but allow
  the full range of different PatchFunction1 and Function1 types.

- add uniformFixedGradient to finite-area for completeness.

Note:
   - still some possible difficulties with the order of evaluation.
   - eg, using an expression within the 'U' field that depends
     of the surface 'phi' field before that is constructed.
     In this case, the 'value' entry is really needed.

tutorials/compressible/rhoPimpleFoam/RAS/TJunctionAverage/0/U

@@ -1,7 +1,7 @@
 /*--------------------------------*- C++ -*----------------------------------*\
 | =========                 |                                                 |
 | \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
-|  \\    /   O peration     | Version:  v2212                                 |
+|  \\    /   O peration     | Version:  v2306                                 |
 |   \\  /    A nd           | Website:  www.openfoam.com                      |
 |    \\/     M anipulation  |                                                 |
 \*---------------------------------------------------------------------------*/
@@ -35,9 +35,19 @@ boundaryField
 
     outlet2
     {
-        type            inletOutlet;
-        inletValue      uniform (0 0 0);
+        // A do-it-yourself inletOutlet (for demonstration purposes)
+        // Need 'value' entry to avoid evaluation (with phi) on startup
+        type            uniformMixed;
         value           uniform (0 0 0);
+
+        uniformValue    zero;
+        uniformGradient zero;
+
+        uniformValueFraction
+        {
+            type        expression;
+            expression  "neg(phi)";
+        }
     }
 
     defaultFaces