Add a limit for cloud droplet effective radius

[szy21]

Right now cloud droplet effective radius is limited with the lower and upper bounds of the lookup table in RRTMGP here: https://github.com/CliMA/RRTMGP.jl/blob/ec0b90dd7aebed03332fea028b4b8844049dac0b/src/optics/cloud_optics.jl#L160, https://github.com/CliMA/RRTMGP.jl/blob/ec0b90dd7aebed03332fea028b4b8844049dac0b/src/optics/cloud_optics.jl#L202.

This is a bit risky, as it could cause issues if the radius is used anywhere else in RRTMGP, and we forget to limit it. We can move it to ClimaAtmos, or find a better place in RRTMGP to clip them.