Adding mean coverage to qc.py

[FredDodd6]

Adds mean coverage to qc.py. And an extra column in the csv for mean coverage.

[m-bull]

Thanks for the PR @FredDodd6.

Tests failed because csv produced by qc.py has to have "qc_pass: TRUE/FALSE" in the final column. The next stage of the nf process uses this column to send samples down different routes.

Should be a simple case of swapping these elements around in the dict.