Groups of samples

[laughedelic]

Similar to the #7 task, but we want to define several groups of samples and see summary results per group.

This probably could be done in some simple way:

• there is summarizing step (loquat) from #7
• instead of passing all samples to it, we can create several data-mappings, one for each group (and one for all of them together if needed)

[laughedelic]

See https://github.com/ohnosequences/mg7/issues/7#issuecomment-217916465