Clean-up and improvements in C++ tools.

- Make gmx-select accept multiple selections for all output options.
- Make gmx-gangle and gmx-distance accept dynamic selections.
- Remove -dump option from gmx-select and remove commented code related
  to the same in the other tools.

Part of #665.

Change-Id: I5e3c4caf2928a95c2620d969b4f447272ad9b051