[WIP] SciDB backend

This isn't ready to merge, but I've started playing around with connecting SciDB to blaze.

There are two failing tests currently (I think one is a backend bug, and one is a SciDB bug). I also want to add some more tests to make sure I understand how blaze is working.
also can you just return something like `compute_up(sqrt(t.child.var(t.unbiased)), x)`? `sqrt` should dispatch on `UnaryOp` and `var` will dispatch on the above implementation