Unwrapping many interferograms leads to large `unwrapped/scratch` dir

[scottstanie]

We're creating the scratch directory for unwrapping in advance, which leads to it getting preserved. this is useful for debugging, but it leads to many GB of tiles/temp rasters when unwrapping dozens. We should add an option (probably the default option) to remove the contents of the scratch dir after it finishes.

Note that we can't simply pass the same scratch to tophu/snaphu-py without making it first, since parallel unwrapping jobs will step on each other and try to delete that directory while others are running. We have to be a bit more careful with just removing the contents, not the directory (or specifying separate scratch dirs for each job. or allowing tophu/snaphu-py to pick a useful tmpdir)