Qualimap BAMQC module minor update

[FerriolCalvet]

Description of feature

In addition to the stats currently reported by the Qualimap BAMQC module I think it could also be very informative to report the proportion of reads mapped inside the regions.
It does not require reading any additional information and is a very informative value that can be computed from the number of mapped reads in the regions and the total number of mapped reads. (in fact it is also reported by the qualimap output)
It is a division of this two, just like the computation of the percentage of mapped reads.
https://github.com/ewels/MultiQC/blob/d7832233f751ed731d7de7b97a195d1e647f0f39/multiqc/modules/qualimap/QM_BamQC.py#L134-L145