Change AntennaAzimuthImage with computed Squint fields

[ACHMartin]

In Oscar/level1

Change the fixed squint reference 45, 135, with the one computed either via python or via MetaSensing pre-processing cf #175.
A sensible name for this field might be SquintImage.

if antenna == 'fore':
ds['AntennaAzimuthImage'] = np.mod(ds.OrbitHeadingImage - 45, 360)
elif antenna == 'aft':
ds['AntennaAzimuthImage'] = np.mod(ds.OrbitHeadingImage - 135, 360)

[ACHMartin]

use the ds.SquintAngleImage

[DavidMcCann-NOC]

Closed with #187