Add field methods that compute forces instead of fields

[leon-vv]

Problem statement

Current methods only return electromagnetic fields. It would be great to have a method that returns forces (with unit N) for a given position, charge and velocity. This would make it much easier for people to write their own particle tracers (for example to take into account space charge, or tracing 'weird' particles)

Proposed solution

Add a method Field.force_at_point(charge, position, velocity) which returns a Numpy array of shape (3,) representing the force

How to test

Add a new file tests/test_field.py with a few simple test cases to ensure the right answers are returned