Refactor SimPSFToolkit.py

[jeipollack]

The organisation of the SimPSFToolkit.py is very complex with various components comprising the class PSFToolkit that could be decoupled e.g. SED-related functions, telescope properties, etc. into their own class objects. Naming of functions and classes could be improved, e.g. its unclear whether poly means polynomial or polychromatic.

It also doesn't seem necessary to have plotting commands at L1009 inside the function generate_poly_PSF.