init_glacier_directories to replace init_glacier_regions

[fmaussion]

Closes #965

[pep8speaks]

Hello @fmaussion! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2020-03-25 12:11:50 UTC

[fmaussion]

I'm going to merge this now - without review. hopefully I didn't brake anything. @matthiasdusch since you are always on the frontline any feedback is appreciated

[fmaussion]

Just some more info:

• init_glacier_regions is back to previous (and consistent) behavior before run_prepro_levels is broken #964
• I tested it in the middle of this PR to be sure I didn't brake anything, but init_glacier_regions is now only minimally tested and not for full functionality (it would have meant duplicating all the tests). So in the future it might get broken without noticing (which is not a biggie since it's supposed to be deprectated)
• so this shouldn't change anything for users in theory. Later we might start to send deprecation warnings (maybe in a couple of months)