Integrate magnetic field into tracking loop

[sethrj]

Support fields, both uniform and nonuniform, in the main tracking loop.

[sethrj]

We should have a geometry virtual interface for calculating the geometric step limitation.

[sethrj]

I was thinking about this today: what we want is one or more AlongStepAction instances, each of which calls a AlongStepApplier<Propagator, MscType, EnergyLoss> (maybe?). To determine which along-step kernel gets applied we will also want to add a selection mechanism to the pre-step kernel (e.g. partitioning tracks into those that have slowing down and those that don't, and assigning "along step action IDs" as appropriate).