phalf, pfull -- monthly and climatological fields oddly placed in tables.

[martinjuckes]

phalf and pfull are at the clim frequency (time2 axis) in Amon, which is not very intuitive. That's something inherited from the original CMIP5 Amon tables. I notice there are several other variables using time2 in Amon

On the other hand, phalf and pfull at the actual mon frequency can now be found in CFmon!

It could make more sense to have:

phalf and pfull at the mon frequency in Amon (even if it slightly breaks compatibility with CMIP5)
phalf and pfull (and some other A variables) at the clim frequency in the Aclim table that we have requested for PMIP, but it currently missing (see #28 )