Enable selection of Ensembl (BioMart) version

[christopher-mohr]

Currently, the Ensembl (BioMart) version that is used for information retrieval of e.g. transcripts is predefined for each reference genome version. I guess it would be better to allow the selection of the version from a predefined list at least.

However, throughout the different versions database field names e.g. changed and might have to be adapted accordingly in the interfaces.