[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.
@mrocklin after this there are a total of 4 or 5 backends whose dispatch on these types look exactly like this, should we consider maybe implementing a generic version?