Atom groups default to not being dummies

Silence additional GCC warnings from John Stone

From this commit on, "system forces" (i.e. total forces minus biasing…

… forces) are no longer implemented, and are replaced by total forces instead. This change is not backward-compatible.

Remove unused variable

Port procs for MW ABF into colvarscript

Also, introduce script-friendly "simple string"
representations of non-scalar colvars. Use them
for gradient callbacks, etc.

Error handling improvements