Unit test for HRRR constraint.

[rcjackson]

Travis CI's 4 GB memory limit prohibits unit testing of the HRRR constraint as a part of the CI. For now I do unit testing using the Hurricane Florence example as a manual pytest on my local machine, but there needs to be a way to test this constraint on Travis CI, especially since cfgrib updated its syntax for loading GRIB files recently.

[rcjackson]

The model constraint is now tested using a fake dataset where U = 1 m/s everywhere. This at least shows us that the model contstraint is working correctly, but the part where the HRRR grid is converted to a Py-ART grid still needs a test.