This updates SOSClient to add formal support for dependency checks.

SOSClient.check_dependencies() now determines if soscmd is

installed.
SOSClient.get_local_path() now calls this instead of checking for

dependencies itself, emitting a warning if dependencies haven't already

been checked via setup() or has_dependencies().
SOS unit tests simulated results, so the test skipping abilities are not

used. New tests have been added to test the SOS dependency check

functionality.

Unit tests pass for Python 3.7-3.11.