Radiation: Transparently add required Properties

[ax3l]

As mentioned here, enabling the radiation plugin for a species should transparently add the required attributes AttributMomentum_mt1 and AttributRadiationFlag without user interaction (if the user did not already add them explicitly).

(Also, the ENABLE_RADIATION define should be removed and replaced with compile-time prettyness #667)

[ax3l]

this is a duplicate of issue #667