Colour calibration to address saturation decrease

Images are affected by decreased saturation towards the edge of each image. When stitching, this necessitates a huge overlap, otherwise we see a periodic loss of colour in the stitched image.

An example of this is in the https://images.openflexure.org/viewer/ biopsies (not smears), and samples with high red intensity (such as H&E stains) are particularly affected

Suggested change could be to get a set of pure red, blue and green samples / gels, and calibrate against the colour response of these