Add HDF5 geometry file format

This MR adds support for a HDF5-based input format in the MeshGraphHDF5 class. Concurrently, it adds support for the use of parallel graph partitioners (presently Pt-Scotch) to permit the faster partitioning of the mesh at simulation startup. The new format is available when compiling with NEKTAR_USE_MPI, NEKTAR_USE_SCOTCH and NEKTAR_USE_HDF5 enabled.

This merge request also addresses a number of small bugs/optimisations to facilitate the larger mesh sizes that this format will enable:

• MeshPartition classes now accept a map of mesh entities, vs. a full MeshGraph, so that we don't need to construct elements a priori before partitioning.
• Various mutexes inside NekFactory, NekManager and MemoryManager have been removed by default and will only be compiled if the NEKTAR_ENABLE_THREAD_SAFETY advanced CMake option is enabled.
• Gmsh 4.0 file format fixes to support reading of the appropriate physical entities.
• Optimisation improvements for NekMesh tetrahedral elements.
• Optimisation improvements for the InputStar NekMesh reader.