Variable (non-orographic) precipitation

[benbovy]

We've discussed with @hannahsophiadavies the possibility to use a field of spatially variable precipitation rate as input. Technically, I think it should just work using the compute_orographic_precip function. However, this is currently not supported in the fastscape extension as OrographicPrecipitation.precip_base only accepts a scalar value:

orographic-precipitation/orographic_precipitation/fastscape_ext.py

Lines 43 to 44 in 6133c97

	precip_base = xs.variable(description="background, non-orographic precipitation rate",
	attrs={"units": "mm/h"})

All it needs is the precip_base variable to accept both a scalar and a 2D field using dims=[(), ('y', 'x')].

Other variables might be concerned too:

• rainfall_frequency could also accept a 2D field? (note: it should be moved to the OrographicDrainageDischarge process - Scaling precipitation to FastScape #4 (comment))
• What about wind_speed and wind_dir? It looks like it would work with 2d arrays instead of scalars just like for precip_base. I've no idea whether or not this makes any sense regarding the theory (model assumptions), though.

Someone interested in putting together a pull-request?

cc @EstebanAce

[benbovy]

Note to @hannahsophiadavies: @EstebanAce's pull-request #6 fixes another issue (precip rates < 0).