Initial MULTREGT support

This PR implements basic support for the MULTREGT keyword. It is on a strict 'immediately required features only' basis:

1. Only the default `ALL` nnc behaviour is supported.
2. Modifying the transmissibility internally in a region is not supported.
3. Defaulting of either region arguments is not supported.

It works by modifying the `TransMult` container, so the consumer API is unchanged. Have worked with this on a low priority basis for quite some time; might still need some improvements but know I really wanted to get some comments.