Replace rescale_gerp rule with gerpcol parameter "-s"

[verku]

Replace the "rescale_gerp" rule with the gerpcol parameter "-s 0.001" in the "compute_gerp" rule, which is doing the exact same thing (dividing GERP scores by 1000). That way, it gets a bit easier for advanced pipeline users to change or remove the scaling parameter if necessary for their project, e.g. if they want to generate GERP scores for another purpose than relative mutational load estimates.

[verku]

Include fixing the documentation about the tree time scale