Test that source code hashing works

[jllanfranchi]

This is critical for transforms that get cached to disk, so we know that they're still valid.

[jllanfranchi]

Initial test: change one line of a comment in pisa/stages/osc/prob3cpu.py; overall config_hash, stage's hash, and distribution makers' hashes all change in the subsequent hypo_testing.py run! Looks good for now, though still need a unit test (which might be tricky). Kicking the can down the road on this one now.

[jllanfranchi]

Further custom ad-hoc testing shows this is working. A full unit test is still necessary, though, but difficult. Removing the tag since this is a stretch goal and doesn't seem to be negatively affecting the physics.