Public API to build a converter incrementally

[kervel]

Hello,

As discussed in linkml/linkml-runtime#244 there are usecases where it makes sense to build a converter incrementally (eg to have an API where not all prefixes are known at creation time): for instance by creating an empty converter and then adding the prefixes and schema with an add_prefix function.

Thanks
Frank

[cthoyt]

Hi @kervel, this seems reasonable (though the PR you linked it very difficult to understand). I'll make a demo PR and tag you for review