Docs Makefile additions, compute/numpy tests.

Fixes the problem of capabilities.py failing to create the graph the first time docs are built on a machine. Simple fix. Also, there are some tests and random changes to compute_numpy in here that I added in. Going to remove the other pull request. @mrocklin @cpcloud 
It differs in that the above one is testing on a numpy array with fields, and the one below is testing on a numpy array without fields. I just realized that the one below actually ends up covering all the  statements in compute_up field, so I'm going to rename it to that.