Name library properly in BWA-MEM

[ihodes]

We currently give them all the same name in the @RG header, same for platform (we should at least have the option to name these correctly, and expose it in the EDSL).

[smondet]

we use the sample-name from th FASTQ pairs which we control already right?
https://github.com/hammerlab/biokepi/blob/master/src/bfx_tools/bwa.ml#L106

[ihodes]

It's the "LB" part of the @rg that's the issue in this case: https://github.com/hammerlab/biokepi/blob/master/src/bfx_tools/bwa.ml#L75