Print warning when referencing an unknown effect in post_effect

Closes #1253
Remove "class" variable from Item definitions

This was a relic from the old way we did loot generation. It is no
longer used or needed.
Allow overwriting vectors in Item/Set definitions
Allow overwriting of vectors in Power definitions
Allow overwriting vectors in StatBlock definitions
Merge pull request #1255 from dorkster/fixes

Add warning for missing effects and remove unused variable
Merge pull request #1256 from dorkster/data_override

Allow overwriting vectors in Item, Power, and StatBlock definitions