rename `avg_unique_mapped_coverage` to `effective_coverage`.

[ctb]

In #211, I 'coined' the term avg_unique_mapped_coverage for the coverage of only the bases that were actually mapped.

then I got curious if I was badly reinventing existing terms, much like we did with detection. so I asked on the microbial bioinformatics slack, and got this nice response from Lee Katz:

I think we call this effective coverage

and using that as google bait I found this paper, "Understand Effective Coverage by Mapped Reads using Genome Repeat Complexity", with the definition:

The effective coverage of a chromosome is defined as the actual amount of bases covered by reads.

which (from text later on) I infer to mean weighted by the number of times the base is actually covered by a read.

So I think we should rename avg_unique_mapped_coverage to effective_coverage.