Clean-up selection poscalc implementation.

Clean-up selection poscalc implementation.

- Replace gmx_ana_poscalc_coll_t and related functions with a C++ class.
  This resolves a few exception-related TODOs, and makes the C++ code
  somewhat cleaner.
- Replace gmx_ana_poscalc_get_type_enum() with a static array, as the
  current uses no longer need to modify the returned array.
- Remove possibility to provide an external position calculation
  collection for selections; it was not used, and the code is simpler
  without.
- Updated Doxygen comments.

Related to #867; it's easier to document the code when it behaves
better.

Change-Id: Ia8d5ba4ad22272b14bdcd521793636892821b140