Horne Optimal Extraction

[Russell-Ryan]

Implement the Horne technique for optimal extraction of 2d prism image to 1d spectra. This will include full uncertainty propagation and bad-pixel masking. This will operate on single prism images, therefore it will also need facility to combine 1d spectra from multiple prism images. This technique requires knowledge of:

1. the sensitivity curve
2. the API for the trace/dispersion implementation.