refactor: improve handling of B classes in main solver

[JOThurgood]

Importing the field-class module to the solver is arguably done in an odd way.

Might be better to simply take magnetic field object as an argument to the ray / swarm constructors , then save that object as a property of the ray. The field should then be available to the solver through a ray property rather than through a separate object in global scope.

Apart from cleaning up the code, for usage this would allow for easier handling of different B fields within the same script.