Make number of training events a telescope parameter

[maxnoe]

No description provided.

[codecov]

Codecov Report

Base: 92.75% // Head: 92.76% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (22ea88b) compared to base (0e4ffea).
Patch coverage: 96.55% of modified lines in pull request are covered.

❗ Current head 22ea88b differs from pull request most recent head b5ddb50. Consider uploading reports for the commit b5ddb50 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2140      +/-   ##
==========================================
+ Coverage   92.75%   92.76%   +0.01%     
==========================================
  Files         216      216              
  Lines       18050    18061      +11     
==========================================
+ Hits        16742    16755      +13     
+ Misses       1308     1306       -2     

Impacted Files	Coverage Δ
ctapipe/tools/train_energy_regressor.py	85.50% <80.00%> (+0.21%)	⬆️
ctapipe/core/tests/test_traits.py	99.49% <100.00%> (+0.01%)	⬆️
ctapipe/core/traits.py	93.53% <100.00%> (+0.10%)	⬆️
ctapipe/tools/train_particle_classifier.py	86.90% <100.00%> (+0.31%)	⬆️
ctapipe/io/metadata.py	96.58% <0.00%> (+1.49%)	⬆️

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[maxnoe]

@kosack @LukasNickel @nbiederbeck comments addressed