Small fix to applying of forces when no atoms are involved.

When there are no atoms involved in the mapping you don't need to
add forces to the atoms duh
Removed data stashing from Vessel.

This basically removes something that doesn't need to be in the base class from
the base class. Keep it simple Gareth
Added facility to restart grids from checkpoint file

Grids can be written to checkpoint so that they can be
restarted