Implement IO with ADIOS2

[JamieJQuinn]

This PR adds integration with ADIOS2 to provide parallel IO. This is primarily for saving:

1. Checkpoints; files with enough information to restart the simulation at a later time, and
2. Snapshots; files containing data for analysis & visualisation.

I'm writing this draft PR mainly as a handover doc for @CFD-Xing. The low-level pieces of the ADIOS integration have been mostly worked out, with the remaining TODOs mainly focusing on bugfixing and the solver-level work of writing and loading from complete checkpoint files.

One useful way to look at the outputted .bp files is using ADIOS's bpls tool, found in build/_deps/adios2-build/bin.

TODO:

• Implement field output
• Optional striding of fields before output (e.g. only saving every 2nd cell)
• Optional lowering of precision of fields before output
• Implement scalar output
• Add unit tests for field output (field on host)
• Add unit tests for field output (field on GPU)
• Fix tests after mesh update
• Investigate possible memory corruption (segfaults every few runs)
• Output initial conditions at start of run
• Include simulation parameters in checkpoints
• Implement snapshot output with settings for striding, precision, steps between snapshots, and which variables to save.
• Implement loading from checkpoint
• Implement removing old checkpoints after writing latest
• Checkpoint main variables, u, v, w and pressure
• Implement time integrator checkpoint procedure
• Investigate which other values require saving for full checkpoint
• Implement sanity checks when loading from checkpoint; are parameters the same?
• Shift/interpolate pressure to velocity grid before saving for simpler analysis (only for snapshots)
• Add setting for number of steps between checkpoints
• Implement file naming for both snapshots and checkpoints
• Test checkpoint files - simulation running from 0-100 should produce bit identical results to another simulation run from 0-50, checkpointed, then restarted and run 50-100.