add representation for measurement control

Explanation of changes

Add representation method for the measurement control.

Motivation of changes

On the MC one can set core elements such as the settables and gettables. But there are no public methods to view them. This PR allows one to show the MC with its key properties.

A possible extension would be to create properties for the items, e.g. something like

@property
def gettables(self):
   return self._gettable_pars

@caenrigen

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),
  • windows tests pass (manually triggered by maintainers before merging).

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For reference, the issues workflow is described in the contribution guidelines.