Sieges can now destroy buildings if enabled by a game rule.

Added Minimal and Slight levels of devastation and epidemic resistance decrease.
Added minimum army size for siege modifiers game rule to change the minimum army size needed to apply the modifiers, with the default of 100 and ranging form 0 to 3000.

Added game rules to change the amount of development decline and epidemic resistance decrease.
Sieges can now decrease the epidemic resistance of the county.