python: adding check for particle existence.

[KaiSzuttor]

Fixes #2150

[codecov]

Codecov Report

Merging #2151 into python will increase coverage by <1%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           python   #2151    +/-   ##
=======================================
+ Coverage      67%     67%   +<1%     
=======================================
  Files         446     446            
  Lines       30095   30093     -2     
=======================================
+ Hits        20284   20288     +4     
+ Misses       9811    9805     -6

Impacted Files	Coverage Δ
src/core/particle_data.cpp	91% <ø> (-1%)	⬇️
src/core/communication.cpp	74% <0%> (ø)	⬆️

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[fweik]

Must be const &.

[KaiSzuttor]

hm... dejavu... we need cython 0.26 for this to work

[fweik]

You can just remove the data member and call get_particle_data in the getters. That is the cleaner solution anyway