[IC] Tests to determine whether the "Regroup" function is working

@martha-thomae this is what you were asking about!

So a couple of months ago I was beginning to wonder whether the "Regroup" part of "Regroup and reclassify" was actually working, because the proportion of neumes actually getting regrouped seemed very low. I made a synthetic neume layer to test the grouping function:

I had squares and I had rectangles, and I also had shapes made of two rectangles placed close together. In the IC, I did the following:
1. Classified one rectangle as "rectangle" and one square as "square"
2. Regroup and reclassify ; All of the two-rectangle shapes got classified as "rectangle"
3. I then classified a two-rectangle shape as a "square"
4. Regroup and reclassify ; The remaining two-rectangle shapes still got classified as "rectangle."

So that test didn't work. I made a new test image that was more fool-proof, to make sure of my results:

This time, instead of putting rectangles side by side to create a new shape, I took a square and put a thin white line through it. This way, the split shape would have exactly the same dimensions as the squares. Then I did this:
1. Classified one rectangle as "rectangle" and one square as "square"
2. Regroup and reclassify ; All of the split-square shapes got classified as "rectangle"
3. I then classified a split-square shape as a "square"
4. Regroup and reclassify ; The remaining split-square shapes got classified as "square."

So that test was a success! It would appear that the "Regroup" function _does_ work, but it's very very sensitive and only works under the best conditions. It's also possible that changing the regrouping parameters might help; in both my tests I just used all of the default parameters. @martha-thomae let me know what you think and I'll be happy to perform more tests if that can be useful!

Start with the IC's "Regroup and reclassify" action and its default regrouping parameters. Reproduce the two synthetic-neume tests described in the issue, then document whether regrouping responds to the classification change and under what conditions. The issue does not name implementation files or an automated test target.

Written by the indexing model from the issue text.