Multithread careful by default (based on stddev), k-mer window is centered rather than left-most, log bias

[seryrzu]

• Careful is recovered. Now multithread and is used by default. Depends on mean and stddev of frequences of unique k-mers rather than just the mean
• Bias term in computation is now a log of diff instead of ratio
• k-mer window is centered rather than left-most
• Unique kmer density is reduced
• Enforced code formatting

Depends on https://github.com/ablab/VerityMap/pull/10 and should be merged afterwards.