Store PDep networks in List *and* Dictionary.

This means we can use the list when we want them to be in the same order,
and use the dictionary when we want to look them up (quickly) by source.

Perhaps an "ordered dictionary" class would be better, rather than us having 
to maintain two things (it'd be easy to not delete from or add to one of these 
stores at some point, especially when pruning)….

This addresses #176