Bugfix and minor cleanup to player::calc_focus_equilibrium
Minor fixes to learning's rng() calls.
No need to check for XP in the pool anymore.
Styling fix.
max_ex < min_ex produces strange results in rng()
SkillLevel::train and _exercise have increased granularity.
Fixed a bug that prevented books from giving experience at 0 skill.
Merge pull request #21 from FunnyMan3595/learning_613_granular_exercise

Learning 613 granular exercise