feature: Soft validators / Safety limits

From a comment by @damazter on #145:

the parser restrictions on setting and getting parameters should only distinguish between valid and invalid option, I think no safety precautions should be set in those options. If safety is an issue, it should be incorporated in the instrument driver (and an overwrite could be implemented there). The safety precautions should not be incorporated in the parameters themselves however, because they should be as general as possible in my opinion.

Seems like we want a second level of parameter validation, independent of and typically stricter than the validators built into the parameters. Sometimes these may also want to be linked to multiple parameters, like Oxford's double-cylinder magnet safety ranges, or "don't allow adjacent gates to have a voltage difference > 2V". How should this work?

cc @AdriaanRol @spauka

Start by reading the discussion in #145 and the existing parameter validators described there. Define how a separate, stricter safety-validation layer should work, including checks spanning multiple parameters and the relationship to instrument drivers; done means the design and implementation requirements are settled.