In 3.0, all variables and model names must be valid python identifiers. This is highlly inconvenient for Vensim models where it is the convention to use white space instead of underscores. This PR adds a flag to VensimModel to automatically map the name of all variables from being a valid python identifier (i.e., with underscores) to being valid for Vensim (i.e., replace underscores with white space).

In addition, it cleans up a whole bunch of technological debt and other minor bugs found along the way. Most notably

• Constants did not subclass Variable
• Variable class now handles name and variable name dynamics
• Constants are now included in calls to run_model so they can be handled by _tranform as well
• A wrong class (Policy vs. Scenario) in sample_uncertainties
• ruff fixes in various places
• adding type hints in vensim.py and model.py
• model name shifts from having to be alpha numerical to being a valid python identifier

closes #393