[Feature Request] Padding utility for field map in case tracking volume is larger than a given field map

Hello,

While trying to use Acts for my setup, I noticed the following:
I am reading in a field map via the `acts.examples.MagneticFieldMapXyz` python binding. However I found out, that this throws an out of bounds error in line 206 in `Core/include/Acts/MagneticField/InterpolatedBFieldMap.hpp` due to the tracking volume being larger than the field map (I verified that these just occur on the edges or outside of the field map data):
`11:40:08 Propagator ERROR Step failed with MagneticFieldError:1: Interpolation out of bounds was requested`
I think it would be a nice addition to give the user the opportunity to pad the surrounding area of the field map with zero vectors in case the tracking volume is larger than the given field map data.
I temporarily solved it by returning a field cell filled with a zero vector in this case, but I am certain there is a much nicer solution to potentially implement this into the framework.