Release 1.6 with CHARMM export in topogromacs

This version contains a few bugfixes and adds support for creating fully functional gromacs topology files for CHARMM force field simulation with code contributed by Josh Vermaas. This specific feature requires VMD version 1.9.2 or newer. For all other features this should also work with older versions of VMD. The use of the 1.9.2 release version is highly recommended because of the many other improvements, though. For users of gromacs, please also install the update for the gromacs-related molfile plugins, as the ones bundled with 1.9.2 have several bugs.