fitsio: do not call f.r.Close in File.Close
fitsio: do not call f.r.Close in File.Close

This CL makes fitsio.File.Close behave more in-line with other
reader/writer wrappers: its Close method won't call the underlying
reader's (or writer's) Close method.
fitsio.File.Close will now only release resources held by File.

Fixes astrogo/fitsio#32.

astrogo/fitsio