Moving PHYS derivations to Phys package

Moving PHYS and PHYSLITE derivations to a new package.

Clean up of the PHYS derivation file to use its own sequence and some tidying to bring PHYS and PHYSLITE even more in line.

So far as I can tell, these technical changes do not affect the output.

@jcatmore , @elmsheus , @tursom , please feel free to check and comment!

This is almost certainly going to conflict once some other MRs go in, so I'll have to clean it up later...