Fixing core reconstruction

[kpfrang]

Discussing with @TarekHC, @mackaiver and @moralejo we found that there was a bug in the calculation of the core position. I've tracked this down to being a wrong sign in one component of the norm vector in the HillasPlanes. In order to not break the direction reconstruction I added two lines which only change the y-component's sign of the norm vector in the estimation of the core position and should not affect anything else.

In this plot right here, you can clearly see that the old reconstruction of the position (blue diamond) was nonsense and the fixed estimate in red is clearly getting close to the true position (black).

[codecov]

Codecov Report

Merging #777 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #777      +/-   ##
==========================================
+ Coverage   71.54%   71.54%   +<.01%     
==========================================
  Files         199      199              
  Lines       10824    10826       +2     
==========================================
+ Hits         7744     7746       +2     
  Misses       3080     3080

Impacted Files	Coverage Δ
ctapipe/reco/HillasReconstructor.py	96.55% <100%> (+0.06%)	⬆️

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[kbruegge]

Nice one. We should check if thats actually a bigger problem. Maybe connected to #721

[maxnoe]

I don't think this should have been merged, as Kai found the underlying reason which was the azimuth rotation, not a minus sign for y.

[kosack]

Ah, that wasn't clear from the discussion. I could revert it, or just wait for a general fix. Which would you prefer?

[maxnoe]

I think it's better to revert, as #778 fixes the problem at the azimuth level. Even better would be to fix it inside the coordinates module, not outside before using the functions.