Refactor map tuples to dicts

[JossWhittle]

Closes #14. Functions which currently return an ordered tuple of maps (lrmap, udmap, cmap, ...) now return keyed dictionaries { 'lrmap': lrmap, 'udmap': udmap, 'cmap': cmap, ... } so that order/usage is explicitly enforced.

List comprehensions over the tuples now use jax.tree_map and jax.tree_multimap to ensure key safety.

@GMW99, this will break the current implementation of the Metrics Callback class which iterates over a zip of the hardcoded map names and the maps tuple. This iteration can be replaced by iterating over maps.items() since it is now a dict already.

[GMW99]

This makes sense as a sensible change. I'll make the required maps.items() change on #15 :-)