Adds `targets` attribute to client Distribution object.
Adds `is_compatible()` to Distribution and maps, client-side.
Refactor _make_intracomm() stuff.
Merge branch 'master' into feature/subset-ranks

Conflicts:
	distarray/dist/context.py
	distarray/dist/tests/test_maps.py
Fix import bug with renamings.
Remove obsoleted method.
Merge branch 'master' into feature/subset-ranks
Refactoring Context comm creation.
Check for compatible distributions in binary_proxy()
Array creation on subset of targets working!

Still have to flesh things out -- this just checkpoints.
context._execute() takes explicit targets.
Add round_trip_counter and tests.
WIP: Fix TestDnpyFileIO with remote cluster

Also refactor it a little.
WIP: Fix TestNumpyFileLoad for a remote cluster
WIP: Pull out the cleanup_file function.
Fix TestHdf5FileSave.test_save_block...

to work with a remote cluster.
Fix TestHdf5FileSave.test_save_3d...

to work with a remote cluster.

Also factor out `check_file` to a reusable `check_hdf5_file` function.
Fix TestHdf5FileSave.test_save_two_datasets...

to work with a remote cluster.
WIP: Factor out engine_temp_path.
Fix TestHdf5FileLoad.