cdk.getSchemes: Allow usage of `registryForScheme`

[stefandesu]

We want to load all vocabulary data in Cocoda from BARTOC in the future (https://github.com/gbv/cocoda/issues/670), but we need cdk.getSchemes for use the registryForScheme method to work correctly. For that, we need a few things:

• [x] (Optionally?) enable the use of registryForScheme for schemes loaded by cdk.getSchemes
• [x] Remove concepts and topConcepts fields from a scheme if the registry derived from the API field was used
• [x] Integrate this properly with the prioritization (i.e. call registryForScheme after merging the results)

If the use of registryForScheme becomes the default (as suggested in https://github.com/gbv/cocoda/issues/670), it is a breaking change and we need to release it as version 3.0.0. However, I was thinking that #40 is also a breaking change and easy to do, so we could include it in the release. I will also look for other necessary changes that might be breaking changes.

[stefandesu]

I decided to make this the default behavior, and with #43 fixed it shouldn't cause issues anymore. This means that the next cocoda-sdk release will be another major version though.