Draft: Example gettables for use with the scheduler

Explanation of changes

Ported over the ScheduleVectorAcqGettable and the StopWatchGettable. So far the only changes are more elaborate docstrings and some style changes etc.

Still TODO:

• Finish writing docstrings
• Add unit tests

N.B. this MR requires the control stack from !70 (merged) to be merged first.

Motivation of changes

Particularly the ScheduleVectorAcqGettable is key in the context of experiment flow planned for v0.4.0.

Still open dicussion points (please chime in):

• Where do we put the gettables? In this initial version they are placed in quantify.scheduler.gettables. Please suggest a better solution here.
• The StopWatchGettable is not strictly a scheduler thing, but it makes sense for it to be in the same place as the other gettables. What do we do with it?
• In the code there is a reference to issue 103, is this still relevant (@adamorangeqs, @AdriaanRol) or can it be removed?

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Merge checklist

See also merge request guidelines

• Merge request has been reviewed and approved by a project maintainer.
• Merge request contains a clear description of the proposed changes and the issue it addresses.
• Merge request made onto appropriate branch (develop for most MRs).
• New code is fully tested.
• New code is documented and docstrings use numpydoc format.
• Changelog has been updated (when applicable).
• CI pipelines pass
  • black code-formatting passes (gitlab-ci),
  • test suite passes (gitlab-ci),
  • no degradation in code-coverage (codacy),
  • no (serious) new pylint code quality issues introduced (codacy),
  • documentation builds successfully (CI and readthedocs).