In UrbanAlbedo: Create a filter for coszen>0 and operate over that

[billsacks]

UrbanAlbedo currently calculates a variable num_solar, and then there is a big block of code that is only done if num_solar > 0:
https://github.com/ESCOMP/ctsm/blob/2632be6af65ad7d1c0d23ae501c76b3cf68d0d6c/src/biogeophys/UrbanAlbedoMod.F90#L264-L429

This logic causes a dependence of some diagnostic fields on decomposition (#17). Moreover, while the current logic avoids doing any work if num_solar == 0, it is less efficient than it could be in the (probably common) case where num_solar > 0 but < num_urban.

So for the sake of both fixing this dependence on decomposition and performance, we should introduce a filter in this routine (which is also passed down to the routines it calls) that holds just the points with coszen>0. Then we should operate over that filter, and remove the num_solar > 0 conditional.

Blocks #17 .

[olyson]

@keerzhang1 was looking at this code recently and brought up this same issue, so I'll take a look at this soon.