Refactor user data directories with platformdirs

[cmutel]

Fixes #100.

The current implementation stores user data in the site-packages directory. This is undesired, as:

• Users should not touch this directory in general
• It makes multi-user installations problematic for many reasons

The alternative is to give each user a cache directory in a suitable location. We follow other Brightway libraries and user platformdirs, the successor to appdirs.

This PR also centralized the location of directory path creation, which should prevent inconsistencies, and provides one clear place where one can find filesystem constants: filesystem_constants.py.

[cmutel]

@romainsacchi @tngTUDOR I would love to get your input on the following:

The current export directory structure could also be refactored, so that the export directory wouldn't depend on the current working directory. Are you open to such a change? I have a solution in mind where it would be set by the user, by an environment variable, or would default to a platformdirs location.

This would also apply to the logs directory, which sits in the export hierarchy.

Is there a reason that iam_variables_mapping isn't part of the data directory?

[tngTUDOR]

The changes are self-evident in code, but it would be worth it to add them to the CHANGELOG

[tngTUDOR]

moves the DATA_DIR related vars to the new filesystem_constants module.

[cmutel]

I changed all the internal imports, and these paths were in __all__ so shouldn't be importable in any case?

[tngTUDOR]

which is the right black formatting ?

[cmutel]

The CI does black auto-formatting, so we can't make this look any different in any case.

[tngTUDOR]

suggestion: let's keep only 1 source of requirements (duplicated in setup.py and requirements.txt)

[cmutel]

Don't forget meta.yaml in conda :) Not sure that one can be anything but duplicative...

[tngTUDOR]

@romainsacchi @tngTUDOR I would love to get your input on the following:

The current export directory structure could also be refactored, so that the export directory wouldn't depend on the current working directory. Are you open to such a change? I have a solution in mind where it would be set by the user, by an environment variable, or would default to a platformdirs location.

This would also apply to the logs directory, which sits in the export hierarchy.

Is there a reason that iam_variables_mapping isn't part of the data directory?

I agree that having a platformdirs folder, is an elegant way of centralizing files.

[cmutel]

The CI failing test is outside the changes I made (pandas variable name). @romainsacchi can I mark it skipped?