Feature: Extend ADERDG to 2D

[bornhoft]

• [x] start new feature branch for 2D ADERDG
• [x] add hexahedral elements
• [x] generalize face id's for arbitrary element shapes.
• [x] add get_elem_ref_from_face_ref function to HexShape
• [x] add get_lagrange_basis_3D to basis/tools.py

[bornhoft]

Finishing convergence testing. Once completed, will plan to push to master.

[bornhoft]

This feature has been completed and added. See pull request for more details.