Clean up vec.h

Clean up vec.h

- Remove cos_angle_no_table() and unitv_no_table(), since they are now
  essentially identical to the normal versions.
- Replace all uses of cos_angle_no_table() with gmx_angle(), since all
  were used in a numerically unstable way.
- Replace uses of m_rveccopy() with copy_rvecn(), which does the same
  thing.
- Move some functions that are not related to operations on rvec/friends
  to other files.
- Remove #includes made unnecessary by the above changes, and add them
  to source files that previously got them transitively.
- Remove extern "C" guards from the header, since it anyways does not
  compile as C.

Change-Id: I933e7257601ee6f0e9ccf516e0bdcb1a85834a51