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This was broken by d62367db, which was some enabling work for
reading geometry info from streams.

Replaced by cell_explorer many years ago

Normally the beam centre is masked out or not on the detector.  However,
when it is, pairing with 000 causes gradients to come out as NaN and the
prediction refinement to fail.

The units needed to be clarified.

It seems that the code wasn't properly updated when multi-crystal
indexing was introducted.  This "continue" statement didn't do anything.

Previously, it was wired to give left handed cells.

See 3fe28041 where it was temporarily removed.