Fix, simplify and test XTC writing

Fix, simplify and test XTC writing

The old implementation had various sins, including repeating setup and
leaking natoms_xtc rvecs of memory per step writing xtc-grps when they
are a subset of the whole system. Cleaned lots of things up.

Added integration-style tests for XTC writing.

Added more grompp -n support to integration-style tests, though not
all of it is actually used by this patch.

Also added a pragma to work around a curious warning by ICC 12 on
Windows.

Fixes #1423

Change-Id: I3a75f890f21e4a87be62bcf877eb5c471670286d