Support neutral charges in the reaction ensemble

[thepith]

Fixes #2374

If all particles have zero charge, electroneutrality is always given.

Description of changes:

• check if any charges are present, before checking for electroneutrality

[codecov]

Codecov Report

Merging #2378 into python will increase coverage by <1%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           python   #2378    +/-   ##
=======================================
+ Coverage      72%     72%   +<1%     
=======================================
  Files         397     397            
  Lines       18765   18765            
=======================================
+ Hits        13571   13594    +23     
+ Misses       5194    5171    -23

Impacted Files	Coverage Δ
src/core/layered.cpp	78% <0%> (ø)	⬆️
src/core/domain_decomposition.cpp	95% <0%> (+4%)	⬆️
src/core/lattice.cpp	89% <0%> (+6%)	⬆️

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