Fixed approximate definition of air used for world volume material

This fixes the approximate definition (80% nitrogen, 20% oxygen) of air within FullSimLigth used as material in the world volume.

When converting a geo plugin to gdml, this definition of air got propagated. When reading the gdml file with Athena, the actual values for air used by Geant4 got overriden, causing large differences in physics.

I checked that geometries with the new numbers are identical to the Geant4 definition of air used in ATLAS:

    <material name="Air" state="gas">
      <T unit="K" value="293.15"/>
      <MEE unit="eV" value="85.5374138686944"/>
      <D unit="g/cm3" value="0.00129000011835775"/>
      <fraction n="0.7494" ref="Nitrogen"/>
      <fraction n="0.2369" ref="Oxygen"/>
      <fraction n="0.0129" ref="Argon"/>
      <fraction n="0.0008" ref="Hydrogen"/>
    </material>

fyi @mbandier