The new patch field mapping class timeVaryingMappedFvPatchField has been
factored out of the timeVaryingMappedFixedValueFvPatchField BC so that it can be
used to map data onto fields stored within other BCs.
In the process the writeEntryIfDifferent function had to be moved from
fvPatchField to dictionary so that it can still be used in the
timeVaryingMappedFvPatchField class and it made good sense to create the
non-conditional variant writeEntry to simplify the patch field write functions.
This rationalisation has been propagated all other patch fields.
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