* experiments: move executor into a module
* experiments: separate executor into base/local
* rename LocalExecutor to TempDirExecutor, run in .dvc/tmp by default
* git: restore HEAD using reset not checkout in detached_head()
* experiments: support calling BaseExecutor.reproduce directly for
workspace runs
* experiments: run single experiments in local workspace by default
* update experiments tests for workspace changes
* experiments: update checkpoints for workspace runs
* update checkpoints tests
* update test_show
* add force param for apply
* exp apply: add --force option
* exp run/res: add --temp option to force tempdir execution for single
experiments
* force apply in non-apply specific tests
* 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
Peter Rowlands (변기호)