Improvements to Gettables and Settables concept

Relates to #126 (closed)

After discussing with @treynders , we have several concrete points that would improve the ease of understanding the Settable and Gettable subjects.

• Rename the Settable and Gettable classes to SettableValidator and GettableValidator as they are only used to validate inputs to the MC and define when settables/gettables are valid.

This would make it clear that there is not some magic conversion happening within the MC.set_settables .

• Add an abstract base class BaseGettable and BaseSettable that can be used to inherit from and should be used in the tutorial example where we create for example the "Resonator" class.

We should still emphasize that inheriting from this is optional, but it would give an "example" class in a way that is commonly understood by programmers. The documentation would also have to be updated to reflect these changes.

@treynders let me know if you have something to add here.