Method get_inner_size

tests/learning_tests/swig_eigen/ZSparseMatrix.h:16–16

        size_t num_rows() const { return rows(); }
        size_t num_cols() const { return cols(); }
        size_t get_inner_size() const { return nonZeros(); }
        size_t get_outer_size() const { return cols() + 1; }
        size_t get_value_size() const { return nonZeros(); }