MWMechanics::DynamicStat: fix members initialization
Calculate NPC health on loading instead of updating it continually

Only NPC with auto-calculated stats are concerned.
Move some levelup logic from mwgui to mwmechanics
Complete health increase on level up
Use the correct formula for NPC health initialization
Fix health calculation at character creation
Merge remote-tracking branch 'potatoesmaster/bug-664'