Update managed carbon densities.

[kanishkan91]

We need carbon densities for Cropland, Pastures and Urban land. We have data for the same from SoilGrids and Spawn et al. along with land cover from ESA. Updating these would need two separate steps,

1. Updating the C code to get land cover for a given year.
2. Updating the preprocessing to make sure we get values for each of the same in the input data.