Add high-order pyramid and prism support from gmsh

Issue/feature addressed

This MR closes #33 (closed), a long-standing issue that was originally in place when gmsh did not support high-order prisms/pyramids. I still have some reservations about the precise support for this within Gmsh, but I am able to at least generate some meshes at high-order containing all element types.

Proposed solution

Prisms and pyramids now have high-order support. Prism elements will reorder interior vertices, but pyramid elements will for now still maintain their original Gmsh ordering. Note that we do not actually use in the output XML or creating geometry objects, or even have support for nodal pyramid points at the moment.

Tests

The cube_all.msh example has been extended to include a P6 version for thorough testing of all edge & face orientations.

Checklist

• Functions and classes, or changes to them, are documented.
• User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).