Curvature calculation wrong

This line needs fixing:
https://github.com/MCFlowMace/CRESana/blob/905d542d533e554f35b246b5c8bd9dedd6bce06b/cresana/bfield.py#L135C1-L136

Check equations. Check that fixing does not break comparison to Kassiopeia. Probably curvature contribution to drift velocity small enough that this error did not matter.