Remove distinction between Detector and Image planes

[andykee]

As part of a broader rearchitecting (and hopefully simplification) of the far-field propagation core, I think it's worth considering removing the distinction between Image and Detector planes.

How are Image and Detector related/different: Detector subclasses Image. Both objects have pixelscale and shape attributes, but while they are optional (default to None) for Image, they are required by the Detector constructor.

How Image and Detector are currently used: Including a Detector object in the list of planes provided to propagate is a trigger for using the DFT to perform pupil to image plane propagations. Image does not currently provide any real functionality.

Proposed change: Remove Detector plane and collapse all behavior into Image plane. DFT-based propagations are triggered by Pupil to Image planes where pixelscale is specified.

Additional things to consider:

• How does this change impact the overall development of an end-to-end propagation strategy
• Put together a table outlining what happens for various combinations of pupil and image planes

[andykee]

Resolved in 6beb781