Fixed bug double adding materials.

[MicahGale]

Description

This fixes a bug where append_renumber can double add a material to data_inputs.

append_renumber does:

1. tries to append
2. if failure occurs
3. change the number
4. append again.

NumberedDataObjectCollection append was doing:

1. data_inputs.insert
2. super().append

This fail and retry method led to data_inputs.insert being called twice. So making sure super().append occurs first fixes this.

Fixes #516

Checklist

• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works (if applicable)