Settings rework

[dgdekoning]

Some initial work on issue #106.

I've added variable typing to the settings classes and implemented functions from the commontasks.py file in the relevant settings classes.

While not quite the same as how sublime handles the settings files (no comments explaining the settings inside the file, etc.) I think this implementation does allow for ease of adding settings later on.

I anyone has suggestions for improvement please add them :)

[coveralls]

Coverage increased (+3.4%) to 47.32% when pulling 6a68431 on dgdekoning:settings-rework into 9eaed7a on LCA-ActivityBrowser:master.

[dgdekoning]

Alright, figured out why those two tests failed: The settings as initialized by the test_settings script are not using the pytest_project project but are instead using the 'empty' default project, meaning the valid_biospheres method specifically fails to find any databases.
Going to leave the alterations like this just to have that edge-case handled.

[dgdekoning]

Closing this as another pull-request has already implemented it.