refactor CStatistics class

savegames incompatible

Most stuff is calculated at turn change now. Iterations over systems
should be much fewer.
Ranks are now calculated so that the rank is higher (worse) only if
the value is actually worse than the better major and not also if
it is equal. This was particularly a problem in the beginning of the game
with moral stats, for majors that come "last" (Omega for instance)
introduce constants for difficulty levels

so that logic decisions can be made based on difficulty
make majorjoining event dependant on human strength
fix humans taking part in majorjoining
make marks calculation more precise

The best mark (1) is given to the best major, the worst mark
(count_of_majors) to the worst, everything else is mapped according
to those. So, if, for instance, a certain major is way ahead in a certain
category, there is now no mayor with mark 2,something but the second best
has mark 4 only (for instance). This is fairer than previously, when the
second best would just get mark (rank) 2 anyway.
show ship capacity in ship tooltip