Flux point method

Implements #50 .
Having a class with name `ImportsExports` or `LoadInputCubes` is a bad idea ... there should be classes representing the objects your code deals with (e.g. `PowerLawSpectrumModel` or `FluxPoints` or ...) and they can have static or class methods to load various formats ... look e.g. at the `TablePSF` or `GammaSpectralCube` I recently wrote for guidance.