Add member functions to MatDense: eq_cols, eq_rows, sum_col, sum_row,...

Add member functions to MatDense: eq_cols, eq_rows, sum_col, sum_row, best_low_rank, row_nonzero_inds, col_nonzero_inds, submatrix.

NOTE: it needs eigen >= 3.4 to compile (slicing support is needed by submatrix()).