simplified imports from external modules; package-level docstring, other docstring edits

Made importing from tincan easier for modules outside the tincan package. Internal tincan modules should continue to use explicit imports. For example, from the outside, you can say:

    from tincan import RemoteLRS, LRSResponse

but inside the tincan package, we have to use:

    from tincan.remote_lrs import RemoteLRS
    from tincan.lrs_response import LRSResponse