Improve handling of fluorescence scans by load_absorption

[gfabbris]

A few ideas:

• Change transmission kwarg to fluo.
• Add default detectors for fluorescence. May be able to do some of this by looking at the detectors used in the scan.
• Add and ROI kwarg? This would be used only if default detectors were used. It would simplify picking the ROI without having to enter a 'detector'
• Create a separate "load_fluorescence" and "load_transmission" functions? It may be the cleaner way to do this. Otherwise a lot of logic might need to go into load_absorption.
• Ultimately, we might want to just read the positioner, detector and monitor from the hints metadata. But we will need to be sure that it is properly done in our scans.