Implemented simulation core and related classes

[epituch]

Continuing the work towards implementing the simulation class...

Original simulation core class is here: https://github.com/nmag-project/nmag-src/blob/master/interface/nmag/simulation_core.py

Notable changes are

• Separated data writing logic to a separate python protocol class and added test for it
• Created mock features file to prevent errors and push off CLI implementation to the future
• Updated quantity class to include some more constants present in the original file
• Updated simulation core class to use modern python abstract class