ACTS-361 Convenience Functions to ease creation InterPolatedBFieldMap

This merge request depends on merge request !331 (merged) (and hence !330 (merged)) and should be merged afterwards. Creating an Acts::InterpolatedBFieldMap::FieldMapper from known grid points and values for the most common grid formats is independent of the BFieldMap file format and hence can go into the Utilities to ease the creation for the user and to avoid code duplication. Fixes ACTS_361.