in the process of adding categorical gradients to kernel_weighted_sum_np
added compute.ocg option to npksum (does nothing at the moment)
added compute.ocg option to npksum

will act like a permutation on the ordered and unordered variables, evaluating
at the minimum category for each such variable.

can be used in conjunction w/ permutation to facilitate simultaneous computation of derivatives of continuous variables and categorical differences of categorical variables