Added growing degree days.
Updated precipitation source
Added Lawrence, KS data
Changed Mx, My, Mz, Mw, Kx, Ky, Kz, Kw mortality and saturation variables to Mseeds, Kseeds, &c.
Changed formatting of leaves and roots dxdt equations
Removed TODO regarding using std::functional
Added code to selectively opt out of re-running an already-run grain.
Should save time during simulations by reducing repetition of calculations