Cleanup task for cldfrc2m (compute_cloud_fraction_two_moment)

During the review of #380, @nusbaume originally wrote:

# 1. Cleanup subroutine names

> I realize we need to keep these names the same for backwards compatibility with CAM, but can we make an issue to rename them to something more understandable once CAM is retired? I am guessing a new user will have no idea what astG_PDF_single is the first time they see it called.

# 2. Reorder so `intent(out)` goes last

> it might be good to also include the suggestion of changing the argument list order of the subroutines so that all intent(out) arguments are either before or after all intent(in) arguments, instead of randomly being in the middle.

# 3. Replace `3.1415927` (2 instances) with `pi`

# 4. Replace magic number 18/28.97 (two places)

> This 18/28.97 fraction looks suspiciously like the ratio of the molecular weight of water over the molecular weight of dry air. Can we add a note to the "future work" issue to use the official constants for that ratio instead of the magic numbers here?

> Also please note that this same future request holds for aist_vector below as well.