Use and check BUNIT all the way through

[teutoburg]

Currently, BUNIT is used somewhat until the FOV HDU output, but afterwards just assumed. While this was more or less fine in most cases, it led to some bugs during #654, that could have been caught quicker. Also, it's certainly useful for the end user to have BUNIT in the output.

Something like:

• Add ph s-1 as a default BUNIT to the ImagePlane and check what's coming in
• Convert from ph s-1 to ph in ExposureOutput or SummedExposure
• Apply QuantumEfficiencyCurve also to Detector, converting photons to electrons (I guess that's where it physically happens?), maybe move the effect into the electronic department
• Check that electrons are coming in at ADConversion
• Convert to adu (lowercase as per astropy's definition...) in ADConversion and write that into the output HDU