- 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.
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