Implemented read/write of TUNITn for flux and correlated flux.
Modified demo program to read referenced OI_CORR tables.
Implemented read/write of primary header keywords in oitable API.
Filter OI_SPECTRUM on CORRNAME.
Fixed read of primary header DATE-OBS.
Primary header keyword support for oifile, oifilter and oimerge.

Added unit test for oifile API.

Calculation of the primary header DATE-OBS when merging has not yet
been implemented.