Zn evaluation at high pH

[yuki-nmi]

The minimum value of Zn evaluation (I_C_ZN) was tentatively set to be 0.4 (#114). This was to prevent that parcels with high pH values, for which Zn concentration is underestimated with the current empirical relationship, receive too low scores (while these parcels seldom experience Zn shortage). This is a temporal solution.

In future, the function calc_zinc_availability needs to be updated, so that the Zn concentration at high pH are properly estimated. This requires more recent observations.