implement bounds checking for user input parameters

[matthipsey]

recently d_pom was set unreasonably by user, leading to hard to diagnose problems

Can we run a check over key parameters, like here:

libaed-water/src/aed_organic_matter.F90

Line 345 in 87d41d8

data%d_pom = d_pom

eg, something like this for particle diamater:

data%d_pom = MIN(MAX(d_pom,1e-10),0.1)

It would be good to standardise this style and approach across the modules.