Load serialized files generated with openmmtools<=0.21.2

[ijpulidos]

When trying to load simulation results from netcdf files generated with openmmtools<=0.21.2 we encounter that the velocities are not present, since they were added in 0.21.3. This results in an error when deserializing as follows

openmmtools.multistate.multistatesampler.MultiStateSampler._throw_restoration_error.<locals>.RestorationError: Attempting to restore from any checkpoint failed. Either your data is fully corrupted or something has gone very wrong to see this message. Please open an issue on the GitHub issue tracker if you see this!

We should be able to read older serialized .nc files with up to date versions of openmmtools. We could just try catching the KeyError exception when attempting to restore the velocities here.

[jchodera]

Catching the KeyError to make this backward compatible sounds like a good idea!