Add possibility for sparse default value different than zero

[icarocd]

In SparseRowMatrix and SparseColumnMatrix, zero is assumed as the sparse value. However, there are many applications were the default value if 1 instead, as in some symmetric distance matrices for example. Although it is possible to handle this limitation by inverting the logic of the application to treat the way the data structure is used, the possibility I am suggesting is simple to implement and very useful.