Spent fuel can be loaded on trains though unit station modules cannot select it

Issue description

Trains can transport spent fuel despite the product cannot be selected in station modules.
Though spent fuel can be selected as a filter on the flatbed wagon when creating a new train.

Reproduction steps

1. connect spent fuel output from a reactor to a unit station module, configured to load
2. wait for some spent fuel to enter the unit station
3. copy-paste settings from the unit station module to the destination module and reconfigure to unload

Game version and build number

v0.7.5c

Operating system

Windows

Interesting, this is tricky. We don't allow radioactive materials because, given the current capacity of our stations, they would quickly accumulate radioactivity. We should either prevent the fuel from getting in or lift the global radiation tolerance.