parsebam.py - added optional arguments to run_pycoverm

[SebastianDall]

Added "None" to optional arguments in parsebam.Abundance.run_pycoverm.

In the logs originating from the bam_abundance rule in the avamb workflow I got the following error:

"vamb/workflow_avamb/src/write_abundances.py", line 30, in
write_abundances(opt.msk, opt.b, opt.min_id, opt.out)
File "vamb/workflow_avamb/src/write_abundances.py", line 15, in write_abundances
(abundance, _) = vamb.parsebam.Abundance.run_pycoverm(
TypeError: run_pycoverm() missing 1 required positional argument: 'target_identifiers'

After adding the default values for the optional arguments and repositioning them, the snakemake pipeline continued.

[jakobnissen]

Thanks for the heads up. I've elected to fix this in the script instead in #202