DEP: get_zerod_matrix removed
MAINT: Table.sample_ids and Table.observation_ids are now of type
np.ndarray
REL: removing backends from setup.py
REL: scipy_sparse removed from extras, hdf5 added to extras
BUG: tocsr/tocsc were being used inplace
BUG: now passing shape to csc/csr constructors
BUG: from_hdf5 was not passing type to constructor
TST: added test for Table._to_dense