Remove default "dry" mixing ratio from constituent object "instantiate"

[peverwhee]

Description

The standard name rules specify that you always need to specify what the mixing ratio is wrt to:

By default, mixing_ratio refers to mass mixing ratios. The long name should explicitly specify that it refers to the mass mixing ratio. Mass mixing ratios should contain information regarding with respect to what quantity they are defined, and options are wrt_dry_air, wrt_moist_air, or wrt_moist_air_and_condensed_water, where moist_air refers to dry air plus vapor and moist_air_and_condensed_water refers to dry air plus vapor and hydrometeors. Use of specific_humidity should be avoided as there is no consensus on whether it refers to mixing_ratio_of_water_vapor_wrt_moist_air or mixing_ratio_of_water_vapor_wrt_moist_air_and_condensed_water. total_water can be used to designate water in every form, i.e. water vapor plus condensed water.

The logic in the constituents object currently defaults to "dry" if nothing is specified, but that should be removed and it should produce an error if you don't specify.