add inorganic phosphorus pools

[jacobcook1995]

I need to add inorganic phosphorus pools to the soil nutrient model.

This will involve restructuring the code a bit. When I do this I should bear in mind @dalonsoa suggestions for improving the code clarity in the soil pools code.

[davidorme]

I agree with @dalonsoa's comments about the dataclasses. I think the attribute dot notation is easier to read and the docstrings on the data class provide a cleaner way to explain the structure of what is being stored.