Made use of VMD plugins more robust

Made use of VMD plugins more robust

Attempt to use VMD trajectory-reading plugins only if plugin
loading functionality exists and shared libraries are being used
(else dlopen can be unreliable).

Also, for helping to find those plugins, the user can supply the plug-in
path in a CMake cache variable. If not, the code can now fall back
on the run-time value of the environment variable VMDDIR, or its
value at configuration time. Previously only an explicit run-time
environment variable or a hard-coded path was available on non-Windows
platforms.

Also printed diagnostic text when GROMACS cannot tell if the
trajectory being read by a plug-in might have atom velocities.

Fixes #598

Change-Id: I44267e115267772dae2c1d754895f8ff25c98a6a