Parallel netcdf

[connorjward]

Fixes #556 and #378

TODOs

• Check that plotting still works (compare to existing examples)
• CI passing
• Cleanup
• Add info to install parallel version to docs
• Remove tweaks to build system
• Final check
• Does the website build?

[tommbendall]

Thanks so much for this! This is a real neatening up and is really going to help with outputting in parallel -- I don't think I'd have been able to do this, so we've been really lucky to have you solve this.

I've followed the install instructions on my local machine, ran a test in parallel and made a plot (with the tomplot change) and all worked. I'd be happy to merge this once we resolve the testing

[tommbendall]

Thanks so much @connorjward, I'll approve this subject to passing tests

Changes made