TransformUnscented throws error for large ensemble size

[odunbar]

Issue

When computing weighted perturbations over the stencil
sqrt(weight) * X .- Xmean, we often compute column one which is always zero.
Though it is not actually use, the weight can actually be negative in column 1 and so can throw an error in such cases.

Solution

Simply compute for columns 2:end (have positive weight) and set column 1 to zeros.