Rename scratch directory and data directory

[mlincett]

Following up on #292 I would propose to rename the following:

• flarestack__data to flarestack_cache or flarestack_workdir; so there is no confusion with FLARESTACK_DATASET_DIR.
• FLARESTACK_SCRATCH_DIR to SCRATCH_DIR, so it is clear that SCRATCH_DIR can be shared since a dedicated directory will be created inside.

[JannisNe]

I like it. It is a breaking change and I would suggest including a version bump to a new subversion already in the PR so that we can create a tag and a new release right away after merging. I guess #292 will come afterward and as such will be a bump to another patch.

[robertdstein]

I agree it's a good idea and just bump to v3