add mfem dof_mapping

[j042]

Overview

this PR removes mfem_index_mapping and adds dof_mapping instead. This supports both 2d and 3d using multi_index

Addressed issues

• 3d missing

Next PR

• tests - io round trip
• extend io for 3d single patch
• multi patch load

Checklists

• [ ] Documentations are up-to-date.
• [ ] Added example(s)
• [ ] Added test(s)