Feature/high order vtu output

Issue/feature addressed

VTK output was manually written, which is slow and results in large uncompressed files

Proposed solution

Adds a VTK output option using the VTK library

Implementation

Renamed old output to OutputVtkLegacy, implemented a new OutputVtk which makes use of the VTK library to achieve a number of things such as custom compression levels 1-9, high order VTK output, multiblock output, cached mesh when using filters.

Supports:

• Old linear: Whatever it was before
• New linear: 1D, 2D, 3D, 3DH1D (cached mesh when filter FieldConvert)
• High order: 1D, 2D, 3D except pyramids (cached mesh when filter FieldConvert)
• Multi-block: 1D, 2D, 3D

Tests

Notes

Please add any other information that could be useful for reviewers. Fixes #215 (closed)

Checklist

• Functions and classes, or changes to them, are documented.
• User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• Newly added files are correctly formatted.
• License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).