Image tests failing with Pint 0.10

[dopplershift]

Saw these with the "build against master" test matrix entry, but thought they were due to xarray--turns out when doing a sweep against pint master I forgot to run the image tests. Oops.

Good news, is that looking at the output of the examples, things work in general. The tests show a few failure modes:

1. Getting negative 0 instead of positive
2. degC -> degrees_Celsius (yuck)
3. Station plot tests giving back text with units instead of just the values (so "20" -> "20 degrees_Celsius")

Not sure if these are truly edge case bugs or just some flaws in the tests...