First take on specification resolution mechanisms

[mjreno]

No description provided.

[mjreno]

This PR introduces mechanisms to resolve out of context input:

• A Resolve type that maps memory paths strings to an existing input object hierarchy
• A load based mechanism to pass params from previously read blocks (Options/Dimensions currently) downwards to a later read context

To test/demonstrate, some incomplete code has been added:

• simulation.py has an incomplete load classmethod intended to temporarily support loading a gwf model. It also instantiates the resolve singleton instance.
• model.py has an incomplete load classmethod inteded to temporarily support loading a model packages block without existing MFList support.

@wpbonelli a hand written example file has been added (spec/ipkg/gwf_ic.v2.py) as a possible next step for generated package python files. In the example, parameter definitions have been moved to a member list and adjustments would need to make to existing objects to accommodate such a change.