Returns handbook value for specified stat for given slot, level and class.
Level can be specified either as number (1-60) or as rank or legendary
specification (R5, L2, E9 etc)
When asking for spellflash handbook value, fifth argument is required
as the spell name and class has to be specific class, i.e. not just "caster".
Please note that spellflash values are in tenths of percent.
Weapon average stat is either 'avg' or 'avg2h', for single-handed or two-handed
weapons, respectively.