Correct LJ tutorial.

[pkreissl]

Fixes #2315

Description of changes:

• Increase number of particles and density. The density was quite low (here, density is a number density -- with the current sigma=1 this corresponds to the rather small volume fraction of ~0.07). Without that, the rdf is not relaxed, to such an extend that it does not capture the expected physical form (always above one ...)
  
  With this PR, we get
  
  where also the number of bins for the rdf histogram is slightly increased.
• skin was defined, but not set in system, now it is.
• The first matplotlib plot did not show after fist cell evaluation. Fixed by giving the import its own cell.

PR Checklist

• Tests?
  • Interface
  • Core
• Docs?

[codecov]

Codecov Report

Merging #2339 into python will increase coverage by <1%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           python   #2339    +/-   ##
=======================================
+ Coverage      71%     71%   +<1%     
=======================================
  Files         381     381            
  Lines       18915   18915            
=======================================
+ Hits        13566   13587    +21     
+ Misses       5349    5328    -21

Impacted Files	Coverage Δ
src/core/layered.cpp	79% <0%> (ø)	⬆️
src/core/tuning.cpp	36% <0%> (+2%)	⬆️
src/core/domain_decomposition.cpp	91% <0%> (+3%)	⬆️
src/core/lattice.cpp	89% <0%> (+6%)	⬆️

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