Spot finding global_threshold auto

[ndevenish]

Apparently global_threshold is only settable as an integer; auto should be an option

[dagewa]

It seems to set fine as a float. Also, I don't find Auto particularly useful here. Who uses it?

[dagewa]

I looked into this but it is more complicated than it should be to set. Having Auto as a special value in the QDoubleSpinBox is trivial, but that doesn't result in it being set appropriately in the PHIL options. We need a way for each widget to interact with its value that provides more control than just str()