Differentiate more precisely bewteen swarm and iterate in PSO more stricly

[kellertuer]

We noticed an inconsistency yesterday, where (probably for historic reasons) the swarm was sometimes returned when calling get_iterate on the ParticleSwarmState – probably since it made a few things easer.

This PR fixes that and introduces :Swarm to access the swarm in general within the (much newer than PSO) get_manopt_parameter and updates its occurrences accordingly.
Furthermore get_manopt_parameter is now as tolerant as set_manopt_parameter! and returns nothing (but does not error) if a state/problem/... does not have a certain parameter.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (40955df) 99.60% compared to head (ff27769) 99.62%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #344      +/-   ##
==========================================
+ Coverage   99.60%   99.62%   +0.01%     
==========================================
  Files          69       69              
  Lines        6372     6380       +8     
==========================================
+ Hits         6347     6356       +9     
+ Misses         25       24       -1     

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[mateuszbaran]

For a generic interface maybe :Population would be better as it can refer to any evolutionary algorithm. Adding CMA-ES to Manopt is on my TODO list.

[kellertuer]

That sounds like a very good idea, to call that generic name :Population – will do that :)