Fix center of mass motion removal with frozen atoms

Fix center of mass motion removal with frozen atoms

When frozen atoms were part of center of mass motion removal groups,
they would still contribute to the mass of those groups. This meant
that the COM velocity correction was (slightly) too small. Now
completely frozen atoms are removed from COM removal groups by grompp.
When atoms are only frozen along one or two dimensions and part of
a COM removal group, grompp now issues a warning.

Also fixed an nullptr or incorrect string buffer passed to warning()
with invalid freeze group dimension user input.

Fixes #2553

Change-Id: I20a03fea511e75a131cb27880acc1f4ee4a2bfb8