Minor clean-up to gmxapi package.

Minor clean-up to gmxapi package.

* Resolve some linting warnings and potential import ambiguities.
* Apply recommended pybind iteration mechanism for Python objects.
* Remove some extraneous syntax and imports.
* Make explicit a call that relied on positional argument processing.
* Alias an imported name (Future) to avoid namespace collision.
* Normalize initialization of DataSourceCollection.
  * Make sure that initialization at construction uses the same setter
    code path as subscripted assignment on the object instances.
  * Add ResultDescription to gmxapi.operation.Future.

Change-Id: Ia687929302edd85a0af616b1d947db21e2f3876e