[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.
SciDB has a three-way distinction between NaN, Null, and empty elements (NaN = a floating point value, Null = a flag whose meaning is user-defined, empty = no data here). So @cpcloud and @mrocklin 's suggestions are technically different numbers