ML FanData is limited to an odd number of detectors

[robbietuk]

This is an summary of the issue discussed in #1499 , specifically from #1499 (comment) 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.

---

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 (#1499 (comment))