[KEY FEATURE] Modify model parameters directly in UI #57

A simplification of PyPSA/temp#10 since it's assumed that the model has already be defined elsewhere and the analyst just has the ability to modify parameter values defined in tables.

Could be considered a duplicate of PyPSA/temp#26 except that it doesn't require the concept of scenarios to be available in the UI, just the ability to edit data of a model that's been loaded in.