Fast colRanks implementation in scran

[LTLA]

... which could find a new home here (src/get_scaled_ranks.cpp). Uses beachmat. Currently only supports rank averaging, but it shouldn't be too hard to generalize it to other ties.method.

As an aside, it seems that matrices returned by DelayedMatrixStats functions are probably going to be dense; so it makes sense to restrict to simple or HDF5 matrices for the time being.