Feat/inspect utils get functions

Explanation of changes

Add new function get_functions to return the functions of a module. Added new more generic function get_members_of_module that is used in get_classes and get_functions.

Motivation of changes

• In quantify-scheduler there is a use-case to list all serializable classes.
• In experiments_transmon there is a use-case to list all possible mock functions of a module.

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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).