Communicate FieldPerp consistently with other Fields

[ZedThree]

This will help simplify the Z parallelisation by being able to reuse the existing Mesh::communicate infrastructure.

• Store a variant in FieldGroup that can handle FieldPerp
• Removes Mesh::communicate(FieldPerp&) overload

A couple of alternatives to this:

• add is2D() and isPerp() virtual methods on FieldData, and implementations on all the Fields and Vectors
• combination of the above and storing Field* instead of FieldData* in the FieldGroup
• move Mesh::msg_len and BoutMesh::pack_data/unpack_data to virtual methods on FieldData

The latter is not great, because they're really details of the mesh, and I wasn't keen on adding more single-use virtual methods -- but now that I've implemented this method, they might've been simpler!

@bendudson Thoughts? This seems "cleaner" in some ways, but definitely also makes things more complicated and slows down compilation