Simplify combat stats by adding StatSet class

The StatSet class is going to be used for holding the values for the
various, persistent stats that are used during combat (hitpoints,
attack, defense, special attack, special defense, and speed).

Aside from the base stat set, an effort value stat set is added to both
the Species and Pokemon classes.

- The effort value stat set in the Species class is going to represent
how the effort value points are distributed after each Pokemon battle.

- The stat set in the Pokemon class is going to represent that Pokemon's
current effort value distribution.

The way that the effort value yield is going to be applied to the
Pokemon's effort value set, is by use of the transform method.

victorPokemon.getEffortValueSet().transform(defeatedSpecies.getEffortValueYield());