Refactor Carsons models to avoid double class creation

[etimberg]

The CarsonsEquations model class is instantiated from another object that contains the necessary attributes. This poses a problem when you want to run Carson's equations on some values as you need to create a shim class to pass those values to the model.

Some ideas for improving this

• add property names in __init__ instead of a plain model
• keep the model and replace with a dictionary