Upgrading the way we deal with single-copy gene collections

[meren]

Although it has been extremely useful, the use of bacterial single-copy core genes has been a limiting factor to identify arcaheal bins easily from the interface.

Here is a complete revamp of the way we estimate the completeness of a given group of splits.

With this change the number of default SCGs for bacteria goes down to two (we retain only Alneberg et al and Campbell et al), and a new SCG becomes available for archaeal organisms (Rinke et al). Mike Lee had made this very important contribution, which in fact convinced us the importance of improving this: http://merenlab.org/2016/05/21/archaeal-single-copy-genes/ --so the anvi'o developers thank him very much for that :)

I am committing this pull request now, but I will extend the technical description of what happened later.