A matrix object can either be an input (i.e. a file on disk or a raw matrix in memory),
or it can represent a delayed operation on one or more matrices. The all_matrix_inputs()
getter and setter functions allow accessing the base-level input matrices as a list, and
changing them. This is useful if you want to re-locate data on disk without losing your
transformed BPCells matrix. (Note: experimental API; potentially subject to revisions).
