Rename average_size to average_intensity

[kbruegge]

I think we call it intensity in most other places. So I renamed it here.

[codecov]

Codecov Report

Merging #953 into master will not change coverage.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master     #953   +/-   ##
=======================================
  Coverage   78.86%   78.86%           
=======================================
  Files         191      191           
  Lines       10966    10966           
=======================================
  Hits         8648     8648           
  Misses       2318     2318

Impacted Files	Coverage Δ
ctapipe/reco/hillas_intersection.py	26.31% <0%> (ø)	⬆️
ctapipe/reco/HillasReconstructor.py	97.29% <100%> (ø)	⬆️
ctapipe/io/containers.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 37f70ce...d04e853. Read the comment docs.