Fix issue with wrong values for overlapping bed-ranges

[Jakob37]

As a part of testing #83, we realized that some values were used multiple times in calculations.

Tracking this down, it seems that in track.rs, same ranges were used multiple times. This is the part of the code I recently updated to fix a separate issue, and might have been a new bug caused by solving the old.

I have prepared a 10nt d4 file mapped with two reads to test this:

$ d4tools show input_10nt.d4
chr     0       3       0
chr     3       4       1
chr     4       7       2
chr     7       9       1
chr     9       10      0

Expected total coverage would be (3 * 2 + 3 * 1) / 10 = 0.9

Before the update:

$ d4tools stat -s mean -r input_10nt.overlapping_ranges.bed input_10nt.d4
chr     0       10      1.2
chr     4       5       2

After the change:

$ d4tools stat -s mean -r input_10nt.overlapping_ranges.bed input_10nt.d4
chr     0       10      0.9
chr     4       5       2

I have prepared a unit test based on this as part of the PR. This and other tests are succeeding, except two remote-view tests (which I don't think are related to this change).

Would you be able to review @brentp ?

[brentp]

let this_begin = top.get_range().0.max(last_end)

[brentp]

let this_begin = top.get_range().0.max(last_end)

[Jakob37]

Thank you for the review @brentp ! Makes much sense to do more careful testing.

I added a more extensive case and simplified the code as suggested.

Ready for re-review.