Frelon peaksearch cleanup

[jonwright]

Quick attempt to use this just now, I will eventually try to send a pull request for these.

• mask should be optional (e.g. None)
• background / dark should be optional (e.g. None)
• flat is missing?
• spline should be optional for segmentation (it comes in with the geometry fitting later)
• we miss a "guess background" function (the old bgmaker.py script)
• frelon_peaksearch is not importable (3DXRD folder name cannot start with 3).
• frelon_peaksearch.worker.bgsub only works for data with a strong background. Otherwise div0, nan.
• ImageD11/nbGui/segmenter_gui.py figsize=(22, is impossible to resize (out of browser viewport)