Copy specified HDF5 group from the ICs to output snapshot files

This tries to address issues #772 (closed) and #556 (closed). On startup the named HDF5 group is copied from the initial conditions file into memory. The group is then written to each snapshot.

There is currently no check on the size of the group so if the group is large or we accidentally set InitialConditions:metadata_group_name to the root group or PartType1 etc we will have problems.