Add initial version of NekMesh generator

This merge request completely renames and reworks MeshConvert into NekMesh, which along with manipulating meshes can now create them.

The mesh generator is in a early version but it does work robustly for a wide range of geometries and has a basic module for producing prism layers.

There are a number of other changes including new modules for fieldconvert and NekMesh and small tweaks to fundamental libraries to improve the linear elastic solver, namely non sequential id meshes.

Things left to do:

• Write documentation for mesh generation