Add `splash` to profiling suite.

[davidorme]

Is your feature request related to a problem? Please describe. With the merging of #69, it would be good to add the SPLASH module to the profiling suite so we can monitor it for performance regression and choke points.

Describe the solution you'd like Take the example analysis in docs/sources/users/splash.md and convert it into a profiling test. That dataset isn't particularly big, so we might want to consider something with a heavier load at some point, but it should be fine for now.

[tztsai]

I'm using the splash_test_grid_nw_us.nc dataset to profile SplashModel as in the example in splash.md. I wonder if a single function calling estimate_initial_soil_moisture followed by calculate_soil_moisture for the whole time series suffices to measure the runtime performance of the whole Splash module.