Feature: user API improvements

[chrisbc]

as API user I want some simple, documented API helper methods that make it easier it easier to extract useful information from the model:

for example:

>>> def get_branch_set(short_name:str):
...     for branch_set in model.source_logic_tree().fault_system_lts:
...         if branch_set.short_name == short_name:
...             return branch_set

>>> for b in get_branch_set('CRU').branches:
...     print( b.inversion_solution_id, b.values )
...
U2NhbGVkSW52ZXJzaW9uU29sdXRpb246MTIwNzIy [dmgeodetic, tdFalse, bN[0.823, 2.7], C4.2, s0.66]
U2NhbGVkSW52ZXJzaW9uU29sdXRpb246MTIwNzE3 [dmgeodetic, tdFalse, bN[0.823, 2.7], C4.2, s1.0]
...

find a similar libraries and select an appropriate pattern from those (agree with chrisbc and chrisdc)

Suggestions:

• improve listing / accessing the models
• improve branch set retrieval (as above).
• help and/or docs for accessing and interpreting the various dataclass attributes. (custom repr() ??)