Avoid "hidden state" of objects

[wtbarnes]

There are several places where methods set attributes or objects or worse, objects set attributes of other objects. This is not good practice and makes workflows confusing. I'll try to keep a list of these cases here and hopefully resolve them one by one.

• Field.configure_loop_simulations sets the simulation type used to simulate loops.
• Field.load_loop_simulations sets the parameters_savefile attribute for every loop in Field.loops
• Observer.build_detector_files sets the observer_coordinate attribute on an instrument if it is None (should change such that when instantiating an instrument, the observer coordinate is required)