- skip loading of fields with -no-internal, -no-boundary
- suppress reporting fields with -no-internal, -no-boundary
- cache loaded volume field for reuse with point interpolation.
Trade off some memory overhead against reading twice.
NOTE: this issue will not be evident with foamToEnsight since there
it only handles cell data *or* point data (not both), so a field is
only ever loaded/processed once.