Various MeshConvert additions

This MR adds various features and corrects quite a few bugs present in MeshConvert:

Features:

• Ability to split hexahedra in the boundary layer refinement module.
• New module to add curvature defined by a scalar function z=f(x,y).
• Improved Gmsh support for high-order elements.
• A small test suite of relevant examples which we can build on later.

Fixes:

• Many fixes relating to face-interior curvature.
• All MeshConvert elements are now orientated exactly in the same manner as their Nektar++ counterparts. (This will allow for some speed improvements in a future branch.)
• Lots of small bugs in InputNek relating to high-order curvature.