Idealised initialisation of temperature and relative humidity profiles

### Are there any linked Issues or Pull Requests?

https://github.com/mo-cjsmith/lfric_apps/tree/t_and_rh_init

#211

### Brief description

For idealised modelling in the Cartesian domain, it would be useful to have the option to initialise using given vertical profiles of absolute temperature and relative humidity. This can be achieved by a layer-by-layer integration. At each layer the hydrostatic balance equation is solved together with the chosen constraints on temperature and water vapour. In this way any combination of absolute or potential temperature, and mixing ratio or relative humidity, may be used for the initialisation. Other options may be added later, eg. using surface temperature and buoyancy frequency to specify temperature.

### Further details of the issue.

The following applies when vertical profiles of prognostic fields are initialised using
[namelist:idealised]
test='specified_profiles'

There's a new setting in the initial_temperature and initial_vapour namelists: profile_variable. In the case of temperature this can take one of two values, 'potential' or 'absolute', which sets the variable that is specified in the profile_data. For instance, the following specifies an isothermal atmosphere with T=300K:
[namelist:initial_temperature]
profile_data=300.0
profile_heights=0.0
profile_size=1
profile_variable='absolute'

Similarly, whether profile_data in the initial_vapour namelist is interpreted to contain mixing ratio, or relative humidity data, depends on whether profile_variable is set to 'mr' or 'rh', respectively. For example, the following specifies an atmosphere of 100% humidity (RH=1.0):
[namelist:initial_vapour]
profile_data=1.0
profile_heights=0.0
profile_size=1
profile_variable='rh'