Rename `specific_humidity`

[MayeulDestouches]

Description

This PR follows discussion in issue #36 about the ambiguous definition of specific humidity. Specific humidity is usually defined as the mixing ratio of the mass of water vapor to the mass of moist air, but moist air includes hydrometeors or not depending on who you are talking to.

This PR modifies rule 5 on mixing ratios:

• The suffixes _wrt_moist_air and _wrt_total_mass are replaced by wrt_moist_air and _wrt_moist_air_and_condensed_water, which make it clearer that moist_air does not include condensed water in this standard.
• specific_humidity is no longer proposed as an alias for water_vapor_mixing_ratio_wrt_moist_air, not to favor this definition over water_vapor_mixing_ratio_wrt_moist_air_and_condensed_water.

This PR also modifies the standard names specific_humidity and specific_humidity_at_2m into their explicit versions water_vapor_mixing_ratio_wrt_moist_air [ _and_condensed_water ] [ _at_2m ].

Issue addressed

• fixes #36

Notes

I haven't replaced the 22 other names that are built on variants specific_humidity. If required, I can replace them by the current CCPP definition which is water_vapor_mixing_ratio_wrt_moist_air.

[climbfuji]

@grantfirl I assume we need updates to ccpp-physics, scm, ufs-weather-model and neptune to reflect this change?