add weight handling into kdcount internals.

This is useful only for monopoles. Supposed to be fast.
thus not very useful.
calculating correlation function

this does not use the weight handling in kdcount.h

dataset can be continuous, (field) or discrete (point)
paircount takes a digitizer as input.

currently two digitize class are implemented.
digitizeR is for monopole

digitizeRmu2 is with direction.

should also add the quadro-pole. How?