Add support for rainfall amount diagnostics

[ukmo-huw-lewis]

What problem does your feature request solve?

Following #1472, it will be useful to add support for surface_microphysical_rainfall_amount and surface_microphysical_snowfall_amount, as a time-aggregated comparison of precipitation fields.

Note in time it will be important to support 'convective' and 'stratiform' equivalents via new PR to enable analysis of total rain from runs with parametrized convection (e.g. GAL simulations).

Here the aim would be to add support for rain_amount processing, including introduction of a new rainfall histogram method based on Berthou et al "contribution to mean". Comparing this with the more standard (RES) histogram will build understanding of relative differences and information.

Describe the solution you'd like

1. Add in support, including relevant callback(s) for time-accumulated rainfall diagnostics.
2. Introduce new precipitation histogram assumed default plot.

Describe alternatives you've considered

Could introduce the alternative histogram via separate PR, but I suggest there will be value in having rate and amount available, to help compare data and methods.