This new constraint type is preferable to the 'empty' type used previously as it
support patch field values for post-processing and other purposes.
The internalFvPatchField operates as a 'zeroGradient' type so that the adjacent
cell values are displayed on the faces exposed by the sub-setting.
The internalFvsPatchField operates as a 'calculated' type so that the internal
face values are displayed on the faces exposed by the sub-setting.
The immediate benefit of this change can be seen when using 'subsetMesh' without
the '-noFields' option to create and write a sub-set of an 'fvMesh' with field
values, now the face values of the 'exposed' internal faces can be visualised.
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