Support LRG transcripts in the position converter

Note that we explicitely only support LRG references as transcripts,
so using c. positioning to convert to/from chromosomal positioning.

Supporting LRG references as genomic referenes, so using g. positioning
can be future work but converting them to/from LRG transcripts is of
course already done by the name checker.

Converting between genomic LRG positioning and chromosomal positioning
directly is not something that can be easily supported in the current
setup of the position converter.