Schema checker: Allow complete checking of schema

[scpmw]

Most important features added:

• Allow checking dictionaries - including dictionaries that are data arrays ("Quantity-style")
• Add possibility for adding extra dimensions to existing data arrays on-the-fly (important for when we have an existing - say - QuantityArray that we want to add an extra - say - time axis to).
• More elaborate type checks: Use typeguard, support numpy dtype sub-types etc