Add a base class for imaging instruments

[wtbarnes]

There are a few other instruments like AIA (e.g. EUI, EUVI, SUVI) where it would be convenient to be able to just change the name and the bandpass of the instrument but otherwise leave the core of the intensity calculation unchanged. To do this, it would be best to just create a base class for an imaging instrument. This could be pretty easily pulled out of the existing AIA instrument class, with options to modify the name of observatory, instrument, etc. and change the bandpass of each instrument.