in cload pairs, use DCIC pairs format by default

[golobor]

Currently, cooler cload pairs requires specifying which columns contain chrom1, chrom2, pos1, and pos2. Yet, the field already has a well-established standard, the DCIC pairs. It would make a lot of sense (and would cover the 90% of use cases) to set the DCIC pairs column layout as the default, i.e. -c1 2 -p1 3 -c2 4 -p2 5.

[nvictus]

Let's introduce a --preset option.