Allow storing of magnetic field vector of variable dimension (was ACTS-352)

Original author Julia Hrdinka @jhrdinka

When testing the new InterpolatedBFieldMap implementation it was found that the FieldMapper internally stores the Magneticfield values in a Vector3D (see). In case the magnetic field is only given in r and z the FieldMapper and therefore the InterpolatedBFieldMap can not be created, because phi is not known at this point. Solving this ticket should allow to store the magnetic field in variable vector dimensions.

Edited Mar 27, 2019 by Moritz Kiehn

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