- this shifts responsibility away from caller to the individual writers
for knowing which file formats are supported and which file ending is
appropriate. When the writer receives the output format request,
it can elect to downgrade or otherwise adjust it to what it can
actually manage (eg, legacy vs xml vs xml-append).
But currently still just with legacy format backends.
- relocated to dedicated foamVtkOutput namespace. Make it easier to
obtain a formatter directly without a foamVtkOutput::outputOptions.
Make the logic clear within outputOptions (avoid previous, cryptic
bit masking). foamVtkOutput::legacy also becomes a namespace instead
of a class. Relocate commonly used things into src/fileFormats, leave
volField-related parts in src/conversion.
