Add mark_runs and compress_runs

[nvictus]

I personally use this all the time. Figured it's finally time to add it.

[nvictus]

Question: which name do we like better, compress_runs or merge_runs?

It basically very similar to merge, but instead of combining all intervals which transitively overlap (thus, sharing a unique cluster ID), it takes the hull of intervals that are (1) back-to-back and (2) share a value in a column (thus, sharing a unique run ID).

[nvictus]

Now I'm wondering if these couldn't just be special cases of cluster and merge.