Move gmx_omp.* into src/gromacs/utility/

Changes:
 - Rename the file to gmxomp.*, analogous to other headers in the
   directory.
 - Replace the copyright headers with the new format, and put years
   there when the files have been in existence.
 - Fix Doxygen documentation to produce something reasonable.  Some
   brief descriptions are still very long...
 - Add extern "C" guards.
 - Don't install the header, as it is only an internal implementation
   detail.

Change-Id: I22d876b333534b783ef0638202939a89ce62e9d9