simplify calculate_soil_carbon_updates() function

**Is your feature request related to a problem? Please describe.**
The function to calculate the updates for the soil carbon pool (`calculate_soil_carbon_updates`) is much too complex, it's 150 lines long and has 17 arguments.

**Describe the solution you'd like**
I should remember to simplify the function. There should hopefully be an obvious moment to do this at some point when I'm adding more stuff to the soil model.

@alexdewar also suggested the following as good generic approaches to this sort of problem:

> 1. If lots of the arguments to this function are just being passed through to one or two other functions, you could just pass in the _results_ of these functions instead and have the caller calculate them
> 2. If some of the arguments would naturally group together, consider bundling them together in a class/dataclass (provided that this function won't be the only user of it)