Speed up scalings

[artlbv]

The scalings part is now one of the slowest parts of the tools (apart from the caching).
It seems to me that the slowest part is the determination of the threshold value.
It would be very very nice if this part could be sped up as this is vital for producing rates and could potentially make the whole chain run within less than 1 hour.

@bonanomi since you developed this - do you have any ideas what could be optimised?