New attempt at implementing `export_cache` fixture

[janssenhenning]

Based on #29

These are improvements on top of the initial export_cache fixtures that were written by me and @PhilippRue to be able to run aiida-fleur and aiida-kkr workflow tests with both aiida v1 and v2.

Notable changes from #29

• load_cache, export_cache and hash_code_by_entrypoint are cross-compatible between AiiDA versions 1.X and 2.X
• Cache path handling changes
  • If the path to with_export_cache etc. is relative, it is either interpreted relative to the default cache dir (see below) or if not specified to Path(request.fspath.dirname) / 'caches', i.e. in the same directory as the test file
  • if the path is absolute nothing changes
• Added flag --export-cache-allow-migration flag, which attempts to migrate archives if they are not compatible
• Added options to config file
  • default_data_dir: if specified caches with relative paths are placed here
  • calcjob_ignored_inputs/calcjob_ignored_attributes: Additional things to exclude from calcjob hashes. Between v1 and v2 for example the metadata option environment_variables_double_quotes has to be ignored
  • node_ignored_attributes: dict, where data nodes (specified by entry point) can ignore additional attributes
• Fixed bug in monkeypatching Calcjob nodes. Here the version was not added to the ignored attributes by mistake

The ignoring of things like environment_variables_double_quotes is not ideal, but I think being explicit with this is better for now.

[ltalirz]

@janssenhenning are you still planning to complete this PR?

[janssenhenning]

@ltalirz Yes I think this would be useful to finish up, since we already use it with a lot of success for veryfying that our plugins work with aiida 1.0 and 2.0

I cleaned the formatting up a bit and added some explanations to the now quite convoluted mokeypatching code What I'm not quite sure on is how and whether I could write more fine-grained tests for these fixtures

As with #57 I have no idea what to do about the test failures

[ltalirz]

Cheers! I'll try to find the time to have a look at this over the weekend

[janssenhenning]

Superseded by #67