Add optional to disable JIT cache about ffcx

[chrisrichardson]

It's a little bit tricky, as we need to manage concurrent runs (on different non-communicating processes). If they share the same output folder, then it could cause a clash. Alternatively, they could create a random tmp folder for each compile, but that could result in a lot of stranded temp files.

[garth-wells]

Could use https://docs.python.org/3/library/tempfile.html to manage temp files, and add process number to signature and a simple step.

[garth-wells]

Related

• #133 for setting the cache directory
• #136

[garth-wells]

To close this issue the option added in #136 need to be complimented by and environment variable.

[garth-wells]

Fixed in #166.