make minspread/maxspread, grassspread PFT parameters rather than global parameters

[ckoven]

Summary of Issue:

Right now the spread parameters, which semi-elastically control the allometric coefficient between crown area and DBH, are global constants. Really they ought to be PFT parameters, with the crown-closure elasticity operating at the patch scale but affecting different PFTs differently, as they may be more or less elastic in their allometry or just have different allometries to begin with. I was going to add this to the set of things I'm pulling out of code and adding to the PFT file on branch master...ckoven:newpftparams but I realized that is a bigger job and impossible to do b4b. So I'm tabling this for now. But just want to note that it is a thing to do, particularly if we want to apply the model to places where trees with vastly different allometries (I'm looking at you, California) compete in a given patch of ground.

[rosiealice]

Just saw this. True, and I think you'd need to have some sort of patch level normalized spread state variable (0-1 controlling range from PFT-specific min to max_spread). Is it possible to do bfb if the PFT parameters are all the same?

[ckoven]

I doubt it, but maybe.

[ckoven]

fixed with #279.