composite bandpasses

The current implementation for handling cases where each sensor has a different QE (Quantum Efficiency) is overly complicated. We plan to re-encapsulate the handling of different QEs within a CompositeBandpass class. This class will manage the combination of (1) a static, radial, or XY bandpass and (2) a collection of QE curves. The CompositeBandpass class would have the same interface as the other bandpass classes, effectively hiding the added complexity of multiple QE curves. This change would significantly simplify the code in both FilterLib and _FluxArgs.