Add missed algorithm for clearing the sparse matrix cell

[vkostyukov]

It could be a 0 (zero) as parameter for matrix.set(). So, we have to remove the existed cell in this case.

Here is the right place to put the implementation

for (int jj = columnPointers[j]; jj < columnPointers[j + 1]; jj++) {
            if (rowIndices[jj] == i) {

                // TODO: if value is zero:
                // clear the value cell

                values[jj] = value;
                return;
            }
        }