Compressed changes

The changes to NED2Subregion and PreHPCC fix substantial, time-consuming problems I had way back when and I do recommend checking them out if you think somebody will really re-implement this toolchain for 33 more states or whatever. The walling script now selects "this" HUC8's walls directly instead of intersecting with the flowlines and resets environments to avoid an extent inheritance issue that popped up.

I've tested them all and I'm fairly sure they all produce the same results as intended but they should be checked again of course. There's also a testing function and workflow now to the first few modules that uses test_tools.py to call the tests without the possibility of leaving test material in the actual tool, don't know about anyone else but I often forgot to revert changes when doing that.