Bugfix, add if clause to check header of bam file.

Line 31 of src/collapse_coverages.py:
Sort strings instead of integers to make the code more general. Now keys can be string or number, when sorting strings we get the same result.

This is an unbiased estimator of within variance as per Schloissnig et al.

Code from Lucas Paoli

These files were lost in the repo transfer...

On a small test set, the extra time for the -div option went from ca. 24 hours
to 1 minute.

Mention in README
Check before import