Made method to auto insert new materials to data_inputs.

[MicahGale]

Description

This makes adding materials a lot more convenient. This is done by making a new subclass of NumberedObjectCollection: NumberedDataObjectCollection. When an item is added to this (anyway, because under the hood append is always used) it is also added to problem.data_inputs. When they are deleted vice versa is true as well.

Items appended try to reflect user preference by being 1 after the last instance of this type in data_inputs. So a new material will be added after the previous final material.

This isn't perfect but users can always sort data_inputs how they please.

Fixes #465. fixes #93.

Checklist

• [x] I have performed a self-review of my own code
• [x] I have made corresponding changes to the documentation (if applicable)
• [x] I have added tests that prove my fix is effective or that my feature works (if applicable)

[MicahGale]

TODO:

• [x] implement testing and look into coverage
• [x] Check documentation update needs
• [x] Decide how to handle deletions.