Transforms a couple of index vectors ir and jc (ij of a sparse matrix)
with possibly repeated values into sparse indexes i,j and a vector of 1d indexes of non zero values.
The response can be then used for repeated creation of sparse
matrices with the same pattern by calling iv2v()
ir and jc are supposed to be sorted in increasing order, column-wise (ic runs first)
ij2ijv_i(ir, jc)A list with fields i, j and iv
An integer vector, row indexes
An integer vector, column indexes