Easier input parameter distributions and load statements for Model objects

[sarangbhagwat]

This adds a new class EasyInputModel that lets users input parameter distributions and the statements to be executed to load values into those parameters by means of pandas DataFrame objects or spreadsheet files.

[yoelcortes]

@sarangbhagwat,

This looks like a fun way to create models. I hope we can work it into the main code a little more. I would suggest the following edits:

1. Remove the subclass, add the new method to the Model class, rename the new method to parameters_from_df.
2. Remove namespace_dict from the model __init__, leave it as an input to the parameters_from_df method and rename it to namespace (considering python namespaces are dictionaries, it think we can opt for a shorter name).
3. Replace namespace default to None (using mutable objects may lead to issues later).
4. Add new feature to autofill the namespace with all objects in the flowsheet (except if name is already taken):

if namespace is None: 
    namespace = self.system.flowsheet.to_dict()
else:
    namespace = self.system.flowsheet.to_dict() | namespace

5. Add a test in tests/test_evaluation.py

Thank you! Let me know your thoughts or if you'd like me to help with any implementation.

[sarangbhagwat]

Closed, instead implemented in #209.