BCC generation and assignment goes specific -> general

[lilyminium]

Unlike other parameters in force field files, BCCs are generated and assigned such that the first parameter matched is used. This is the opposite direction to other force field parameters, where the last match is used. It would be convenient to swap to the conventional direction. If we do this, we should probably give plenty of warning so people who may have saved and use custom BCC files aren't caught out.

[mattwthompson]

Somehow I wasn't watching this repo - this seems like a good idea