Create system from Desmond files

I built this reader for validating the gromacs charmm22star ff, but it could be of general interest. It also gives access to charmm36 (re: #36) in OpenMM, if you set up the system with `viparr`, which is as simple as

```
viparr input.pdb system.dms -f charmm36_ions -f charmm36_aminoacids -f tip3p_charmm
```