Add Nek5000 mesh and field conversion

This MR adds new modules for NekMesh and FieldConvert to support input from Nek5000 based format, which is a tensor-product GLL hexahedra and quadrilateral code. The mesh format is similar to the Nektar rea format but is sufficiently different to warrant its own input module. This MR also adds support for zlib-encoded input and output for NekMesh to reduce file size of test files.

There are some caveats:

- Field input is restricted to that written by Nek5000 in parallel - serial output is not supported.
- Mesh files only support very basic boundary conditions; walls and periodic conditions (there is no support for the `.usr` files that define time-dependent BCs). 

See merge request !680