Module pqcost

Splits the gencost variable into two pieces if costs are given for Qg.

Checks whether gencost has cost information for reactive power generation (rows ng+1 to 2*ng). If so, it returns the first ng rows in pcost and the last ng rows in qcost. Otherwise, leaves qcost empty. Also does some error checking. If on is specified (list of indices of generators which are on line) it only returns the rows corresponding to these generators.