WIP: Comments.
WIP: Change COO empty matrix creation from size 0 dense matrix.

coo_matrix now makes an 1x1 empty sparse matrix when given a size 0
dense matrix. This is to resolve issue when indexing would return an
empty matrix.
WIP: Remove slicing with step != 1 bug.

Bug only occured when col index was nonzero too.
WIP: Correct slicing test to skip np.array(-1) indexing bug.
PEP8
WIP: Remove error, from DOK test.

DOK raises a TypeError in this case, but the test expected a IndexError.