Added support for OI_VIS VISREFMAP column to file-level API.
Improved code modularity.

Over-long functions to read and write OIFITS tables have been broken up.
Added filtering of OI_VIS optional columns to oifilter.

The new (optional) vector columns (VISREFMAP,
RVIS/RVISERR/IVIS/IVISERR) added to OI_VIS in OIFITS v2 are now
handled correctly when filtering.

Over-long functions have been broken up.