ENH: Remove unnescassary fields from halo metadata

Minimize the metadata that is communicated for each halo, since,
only the process that writes the halo needs to know the velocity,
center, etc. of the halo.
ENH: Remove no-op periodic transfrom

Removed the no-op periodic transform since DIY does not need that
to be defined for sending data across periodic boundaries and since
when we send halos to periodic neighbors we don't need to map them.