Stone resource too broad; split into fine stone and rocks

[dzfischer]

The Stone resource is used to represent two different things: (1) large quantities of high quality stone as a building material vs (2) having any little pebbles around at all. Based on (1), Stone is a requirement for many building many Prehistoric/Ancient wonders like the Stonehenge and should be relatively rare. Based on (2), Stone is a requirement for building very basic things like Counting Stones and Quern, and should be extremely common. The issue is that (2) should be much more widely available than (1): coastal or riverside cities should generally have no problem finding some pebbles, without necessarily having access to great building material for massive projects. As an example of the issues this can cause, my civilization that is adept at metalsmithing cannot produce flour due to lacking any "Stone", despite having 8 coastal cities built next to rivers with hills nearby. A nice approach to solve this I think would be to treat Stone the same way Clay is already treated by the mod: there is a difference between the commonly available but less powerful "Clay" resource and the much more rare resource used in the construction of more sophisticated projects called "Fine Clay". I suggest similarly differentiating "Pebbles" (common like "Clay") from "Stone" (rare, high quality like "Fine Clay").