Feature / Let measurement class register a ParamSpec

There is no way (exposed by the API) to register a `ParamSpec` directly with a `Measurement` object. That's a petty, since doing that is necessary in many situations.

Let's fix that! I suppose the only question is: do we extend `register_custom_parameter` or make a new `register_paramspec`?

@QCoDeS/core @cgranade

Start with the Measurement API and its existing register_custom_parameter entry point, then trace how ParamSpec objects are represented. Done means the public API supports registering a ParamSpec directly, with the choice between extending the existing method or adding a new one settled and the behavior covered by tests.

Written by the indexing model from the issue text.