If for some reason one of the radii is not defined SaveNexus crashes
because we access the value without checking it first. We should just
check that both radii are present, then write them out.

(cherry picked from commit 5e7dc01597680694128a3897d2244238f17e145a)

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DOES NOT BUILD

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>

This involed creating a new sort type that is used internally. Then the
code is pretty simple.