indexing allows exprs
add matrix block
import matrix block
Block.on_diag
Block assumptions
Block docstring
Block handles single dim inputs
slicing in matrix expressions
test_args for Block
blockcut
slicing with steps
Block -> MatrixSlice
str and latex printing for MatrixSlice
pretty _print_MatrixSlice
handle slices better in MatrixSlice printing
out of bounds checking on slices
blockcut test
MatrixSlice._eval_transpose if symmetric
with statement in test_slice
Merge pull request #1782 from mrocklin/matrix-block

Matrix slice