Split genbox into 'solvate' and 'insert-molecules'

All working old functionality is retained in new 'solvate' and
'insert-molecules' programs. Registered genbox as obsolete, so someone
using gmx genbox gets a hint, which is also triggered by the old
symlink. No new symlinks, of course. Updated all mentions of genbox in
the whole repo.

For gmx solvate:
* Cleaned up unused functions
* Cleaned up unused headers
* Cleaned up useless command-line options
* Updated documentation
* Checked that its behaviour for writing velocities is OK (but there's
  no good way to test that)

For gmx insert-molecules:
* Cleaned up unused functions
* Cleaned up unused headers
* Cleaned up useless command-line options
* Updated documentation
* Added tests, worked out that -vdwd and spc216.gro were not required
  for a decent test and updated accordingly
* Renamed -cs to -f for consistency with other tools. It would be nice
  to have a hidden fallback so that -cs does the right thing, but the
  machinery does not exist.
* Added helpful check now that -ci is required (generic file I/O error
  previously)
* Fixed bug with gmx insert-molecules -ci and velocities. Old version
  would dereference v pointer because the insertion function does not
  return velocities for the inserted molecules.

Split the respective test cases into separate compilation
units. Consolidated functions called by both new tools into
read-conformation.[ch]. The naming and use of those functions are
horrendous, and will be fixed in the next patch.

genbox-addconf.[ch] is no longer specific to any tool, so renamed it
back to addconf.[ch].

Change-Id: I7781c3db189f5d95d5731e4e86cc0585f7d677c7