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.