- extract progressbar functions to separate class;
GaussianGen class:
- add method to access RNG w/ uniform distribution;

Cluster class:
- add avg.speed measurement during some period;
- add uniformly distributed seeding;

simulation:
- add new params to config;
- add setters for params, on which other params depend (D_phi, D_E, h,
  etc);
- add investigation of avg.speed dependency on angular noise intensity;
- measure avg.speed in a moment after relaxation, don't accumulate
  during observation time;
- fix integration procedure: differentiate previous and current states;
- add spatial partitioning (draft, not tested);
link: http://gameprogrammingpatterns.com/spatial-partition.html
- add unit testing w/ googletest;
- fix Grid-based disc speed calculation;
- add to Grid method update_for_search(), which updates velocities of
  particles;
- update unit tests for new place of velocities;
- add more unit tests for Grid;
- fix Grid, Point, Particle classes;
- remove debugging scaffolds;
- fix compiler warnings;
- modify .gitignore;
- attach Grid to simulation process;
- add config parameter 'integration.use_grid' to decide will be used
  Grid or not;
- add note to Documentation;