Acts-354 Enable access to grid information of InterpolatedBFieldMap

In the current implementation of the InterpolatedBFieldMap no information about the underlying grid can be obtained. This merge request enables access to the number of bins, the minima and maxima of each axis of the underlying grid. The information can now be accessed over the Fieldmapper (a convenience method to access it was introduced) of the InterpolatedbFieldMap which gets the information from the underlying grid. Because both the FieldMapper and the Grid could be replaced by another user implementation (using boost::any) their concepts have been adapted accordingly for consistency. Fixes ACTS-354.