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 
Sort keys are unfortunately poorly defined.  Currently they can be either

1.  A string representing a field name
2.  An iterable of strings representing a sequence of field names
3.  An Expr on which to sort

To this extent a check like this might raise errors that you don't anticipate.  I suggest sticking with the more generic check and response until this gets cleaned up (which may not be for some time).