Force fractional exponent arguments to be 0 minimum

[AlexisRenchon]

Fractional exponent will error if the argument is negative.
In ClimaLand.jl, we have such fractional exponent in src/standalone/Soil/Biogeochemistry/co2_parameterizations.jl for example. We should add a warning if the argument is negative (it should not happen).

See closed PR #841

We may need to discuss more what is the best approach, and where this is happening.

below is the error returned by Julia:

julia> (-1)^0.5
ERROR: DomainError with -1.0:
Exponentiation yielding a complex result requires a complex argument.
Replace x^y with (x+0im)^y, Complex(x)^y, or similar.