Load "a b ps p0" hybrid pressure formula terms.

This PR enhances the current auxiliary factory loading scheme for `atmosphere hybrid sigma pressure coordinate` data to correctly interpret the variant CF formula terms `a: var1 b: var2 ps: var3 p0: var4`.

This complements the current capability to interpret the formula terms `ap: var1 b: var2 ps: var3`.

Where `a` is vertical pressure and `p0` is a scalar reference pressure.

Reference http://cfconventions.org/1.6.html#dimensionless-v-coord.