RadiationMapsMaker: added maps of mass fractions for all natural elements

each grid point will collect the density- and step-length-weighted mass-fraction of the encountered elements. This gives the effective atomic composition of materials in each grid point. The first and last element to track is configurable - by default all natural elements ( 1 <= Z <= 92 ) are enabled. Example output in the service area of LAr from a geantino scan in the barrel: H: 0.00525763 C: 0.162227 N: 0.0470331 O: 0.0583856 F: 0.0808482 Si: 0.000496895 Ar: 0.000285226 Mn: 0.000496895 Fe: 0.0486957 Cu: 0.596274