Use a Quantity-safe assert_allclose

On my Mac with the latest dev Astropy and Numpy, I was getting the:

```
E               UnitsError: Can only apply 'subtract' function to dimensionless quantities when other argument is not a quantity (unless the latter is all zero/infinity/nan)
```

error. I've now made it so all Numpy assertions have been changed to a single version of a Quantity-safe assert_allclose (not that assert_array_almost_equal should not be used as per its docstring). I'm now seeing genuine failures though. Does anyone else see any?