ML FanData is limited to an odd number of detectors

[robbietuk]

This is an summary of the issue discussed in #1499 , specifically from https://github.com/UCL/STIR/pull/1499#issuecomment-2330236446 onwards.

This is an issue for scanners with an even number of tangential positions because the fan data does not account for the min_tangential_poss() element when applying norm factors. The resulting efficiency/ normalization values in min_tangential_poss() positions will be 0 or +inf, which will lead to numerous problems.

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The fix would be to extend the fan data to allow for an even number of entries, but this code issue is spread throughout ML Norm (https://github.com/UCL/STIR/pull/1499#issuecomment-2332808262)