* experiments: Allow branching from checkpoints with modified hyper params
* experiments: initial checkpoints tree functionality for `exp show`
* go back to flat/sortable view
* add test for resume w/modified params
* tests: move experiments fixtures into conftest
* add show checkpoints test
* test checkpoint table rendering
* remove dead code
* test show for checkpoint branches
* experiments: refactor regular/checkpoint experiment separation
* fix scm_checkout pull bug
* experiments: return stages from executor.reproduce
* since executor has to move to threadpool execution anyways, we no
longer have to work around the pickle repro result limitation
* refactor new/resume behavior
* refactor checkpoints tests
* use -f to force checkpoints reset/restart
* checkpoints: use 'exp run' and 'exp res[ume]' commands
* stage: all outs can be missing for checkpoint stages
* regular outputs may not be written at the time a checkpoint is
generated, but subsequent checkouts of that stage should not fail due
to the missing outs
* commit stage on checkpoint generation otherwise non-persist cached outs
are lost
* checkpoints: wait for stage proc to end after killing via terminate()
* stage: fix issues with killing checkpoint stage
* refactor signaling (use threading.Event instead of Condition)
* fix windows bug (kill all child processes via taskkill)
* experiments: refactor executor collection
* tests: move checkpoint tests into separate module
* refactor experiment tests to use stage fixture
Sami Jawhar
Peter Rowlands (변기호)