Dynamically add vars to a State

[Lendemor]

Description:

Add a new classmethod add_var to pc.State allowing to add dynamically a var to a State, that can then be used in the same way as a var defined statically.

To add a variable called "dynamic_var" of type int with default value 42:

State.add_var("dynamic_var", int, 42)

This variable can then be accessed in two way :

State.dynamic_var # access it the same way you would a static var
getattr(State, "dynamic_var") # access it like this if the name of the variable to access change at runtime

Closes #303 #346

Note: I added some basic tests for the new method but they could probably be improved.

Checklist:

All Submissions:

• [x] Have you followed the guidelines stated in CONTRIBUTING.md file?
• [x] Have you checked to ensure there aren't any other open Pull Requests for the desired changed?

Type of change

• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

New Feature Submission:

• [x] Does your submission pass the tests?
• [x] Have you linted your code locally prior to submission?

Changes To Core Features:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] Have you written new tests for your core changes, as applicable?
• [x] Have you successfully ran tests with your changes locally?