Oil rigs & oil fields

Inspiration: https://www.reddit.com/r/Openfront/comments/1qrbpft/gameplay_idea_mining_resource_nodes/

The idea is to convert factories => oil rigs. When the game starts oil fields are placed randomly on the map. Oil rigs can only be placed on an oil field. This creates strategic points of interest as players will fight to control oil fields.

oil rigs will act basically how factories do now expect they can only be placed on oil fields. No new "oil" resource will be addded.