Sphinx: fix code samples (instantiation and naming of System())

[pkreissl]

Description of changes:

• lb.rst contained outdated code for instantiating the System-class, namely missing the mandatory box_l parameter, only trying to set it afterwards.
• Fixed that there and in all other places that I found via a quick search.
• While at it, I renamed the occurrences of instances of System that were not called system, to match the naming convention of consistent style in samples #1845.

[codecov]

Codecov Report

Merging #3542 into python will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           python   #3542   +/-   ##
======================================
  Coverage      87%     87%           
======================================
  Files         536     536           
  Lines       24177   24177           
======================================
  Hits        21056   21056           
  Misses       3121    3121

Impacted Files	Coverage Δ
src/core/particle_data.cpp	94% <0%> (-1%)	⬇️
src/core/electrostatics_magnetostatics/p3m.cpp	85% <0%> (ø)	⬆️

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