Fixing CharGene mutation + demo_string optimizations.

MendelOrganism was much slower, switched it to Organism,
as it doesn't seem to be showing any MendelOrganism features.

Removed float example, it's used in multiprocessing nevertheless.
Too many similar examples with the string in general.
WARNING - CHANGES BEHAVIOUR: New generation handling

- population was creating 2*childCount children instead of childCount.
  Now the behaviour is correct (according to parameter documentation),
  but your code needs to increase childCount twice to keep the same
  behaviour.

- numNewOrganisms - added whole population, instead of X organisms.
  at least in some cases.
New example: demo_case.py

Shows why genetic algorithms works.

Few fixes in organism code.