Add an iterator class to be used with sorting::makeSorted that wraps multiple pointers.

[corbett5]

CRSMatrix<double> m;

INDEX_TYPE row = 0;
INDEX_TYPE nCols = 5;
INDEX_TYPE * columns = ...;
double * values = ...;

// Sort columns preserving their relation to values.
sorting::MultiIterator<INDEX_TYPE, double> it(columns, values);
sorting::sort(it, it + nCols);

m.insertNonZeros(row, columns, values, nCols);