Add positionRange as request parameter to /variants

[patrick-koenig]

In V2.1 the call /allelematrix has the request parameter "positionRange" that can be used like this: &positionRange=referenceDbId:start-end.

But the /variants call is currently lacking this very useful request parameter. It would be really helpful if /variants would also have support for "positionRange".

This would allow to request /allelematrix for allele calls and /variants for reference and alternate alles in the same style and in parallel and not one after the other and in different styles *.

The same applies for the parameter "positionRanges" that is present in /search/allelematrix but not in /search/variants. The latter call should also be extended with "positionRanges".

* I mean using lots of variantDbIds extracted from the result of /allelematrix and use them to request the variants with /variants.

@zrm22