rename the `az_za_simple` interpolation function on UVBeam to be explicit about using RectalinearBivariateSpline

[bhazelton]

Now that there's another (similar) option, should be more explicit about the name. This change needs to be done with deprecation.

Maybe we should also consider changing the default interpolation method for az_za pixel coordinates to use the faster az_za_map_coordinates option?