Added preliminary OIFITS version 2 support to file-level API.

The file-level API (oifile.h) now supports the new OI_CORR, OI_POLAR,
and OI_SPECTRUM tables. Cross-referencing from existing tables using
the CORRNAME keyword has been implemented.

Duplicating OI_VIS tables now copies the new complex visibility columns
(e.g. RVIS).