Customizing the threshold used to keep a protein in the core proteome

[cfrioux]

So far all proteomes must have a protein in a cluster so that the cluster is kept for core proteome computation after MMseqs2.
It seems reasonable that a threshold could be used here to account for the incertainty and noise in the proteomes.
That threshold would be an option to esmecata clustering.

[ArnaudBelcour]

Added in #5.