Consider optionally allowing named segments as evaluator parameters

[tohrnii]

Now that we have made evaluators more general in terms of segments it can accept, it may be helpful to optionally allow named segments in parameters. For example, consider a trace with 5 segments (segment_0, segment_1, segment_2, segment_3, segment_4) and an evaluator that takes first and last segment.

Something like ev evaluator([a, b], [], [], [], [c, d]) is probably prone to error. We should also allow users to do something like:
ev evaluator(segment_0: [a, b], segment_4: [c, d]).

This issue is not of high priority though.