Add in-battle stats

[kipyin]

Currently, the stats (Pokemon.stats) are so-called "Permanent Stats" (see Bulbapedia entry), and they should only change when a Pokémon levels up, or taking a Vitamin. We need to add another attribute to Pokemon to manage the in-battle statistics, which are all 6 permanent stats + Evasion and Accuracy.

Implementing the in-battle stats should be easy, just add a BattleStats class in pokemaster.stats module:

@attr.s(auto_attribs=True, slots=True)
class BattleStats:
    hp: float
    attack: float
    ...
    accuracy: float
    evasion: float

More methods can be added as we implement the battle engine.