Compress when creating a CSCmatrix

[yixing312]

In the original code, the matrix was not properly compressed. In particular, using the 'from_column_iter_dense' and 'from_row_iter_dense' functions to create a new matrix containing 0, the created matrix does not match the result created by 'slice.into()'.

[CLAassistant]

All committers have signed the CLA.

[ebarnard]

The function calling this code is called from_row/column_iter_dense and documents that it Creates a dense CSC matrix. This might be better in a function called something like compress_zeros, or separate methods without the dense suffix.