Allow subsetting of intra + inter matrices

[cmdoret]

Currently, hicstuff view allows to subset either an intra-matrix for a continuous region, or an inter-matrix between two discontinous regions.

@aurelepiazza suggested to add an option to subset all contacts (intra and inter) for a number of discontinuous regions in a whole map. For example:

hicstuff view -f frags.txt --multi-region 'chr1;chr3:1-100kb;chr8:100kb-400kb' mat.tsv

To obtain a map with all interactions between and within three regions on different chromossomes.