Feature/boundary layer generation upgrade

This branch brings the first update to the Nekmesh mesh generation system.

The primary new feature is a complete overhaul of the boundary layer generation capability meaning boundary layers can be generated on much more complicated CAD than the convex only requirements of the old system.

It also adds new documentation to Nektar describing how to operate the mesh generator.

There is a modification to the nodal triangles in the reference elements to make the orderings consistent with what people believed nektar used. This will break some peoples meshes if they use NodalTriFekete or NodalTriElec curve face tags.

Regression tests have been added for the mesh generation.

Completed:
+ Boundary layer upgrade
+ Nodal triangle edit
+ Regression tests
+ Finish docs

See merge request !576