Trace groups need to be named

[vreuter]

As of v0.11.0, we have a new section of the imaging rounds config file; here's an example:

"mergeRulesForTracing": {
        "discardRoisNotInGroupsOfInterest": true,
        "distanceThreshold": 2000,
        "requirementType": "Conjunctive",
"groups": [[79, 80, 81, 82], [83, 84, 85, 86]]

The "groups" encode which timepoints are traced together, but we have no name for each group/structure. The value of "groups" should become a key-value mapping, with key being a name for the group/structure, and value being the list of timepoints as currently.

Required for #382