Remove use of deprecated getField signatures in AtlasStepper and EigenStepper (was ACTS-335)

Original author Hadrien Benjamin Grasland @hgraslan

For historical reasons, the ConstantBField provides three different ways to query the magnetic field at a certain position. The InterpolatedBFieldMap, however, only provides one way to do it: pass a position Vector3D as input, get a field Vector3D as output.

This getField signature should be used in the Atlas and Eigen steppers in order to allow realistic stepper testing based on an interpolated magnetic field map.