Remove custom path management

[cmutel]

settings and settings_wizard has a lot of code around path management. bw2data now has projects.change_base_directories, and this should be used as much as possible.

• change_bw_dir has code around switching to a new directory with a projects DB. This can be removed, we always create a projects DB with projects.change_base_directories
• settings.get_default_directory() is remembering if an environment variable was set. This can be replaced with explicit remembering of the default directory when the AB was started (this will come from the envvar if available, but also work otherwise), and this function should be replaced with bw2data.projects._base_data_dir

[will7200]

This can be replaced with explicit remembering of the default directory when the AB was started

Would you clarify on this? get_default_directory returns either an env that is set by the user, named BRIGHTWAY2_DIR or the default data dir from bw2data. Not sure what needs to be remembered since it will default to bw2data default directory anyways.

@cmutel