New stat function: % of positions in intervals with coverage >X

[Jakob37]

This is something we are looking at over at the https://github.com/Clinical-Genomics/chanjo2 repo.

The current approach is to read the output from the d4 file using d4tools show. It would be much quicker to do this directly in d4tools.

The execution could be something like:

$ d4tools stat --stat perc_cov "10,20,30" --region "intervals.bed"

The desired output would be something like:

chr     start           end             10x  20x   30x
19      31030023        32034023        98.3 96.3  94.1
19      49624990        49625000        94.2 90.3  85.3
19      49625010        49625020        88.5 80.3  75.3

Feedbacks on this are welcome. If I have the time, and no one else jumps on it, I'll give it a go and see if I can implement it.