Add a function to check if lat/lon points are within the WRF domain

Had a user on ncl-talk try to use wrf_user_ll_to_ij to check if observations are within the WRF domain. The current ll_to_xy routines are simple lat/lon to x/y converters that don't care what the domain size is. A new function should be created to add a check to see if the points are outside of the domain bounds. Not sure how the return should be -- either an array of booleans, or sequences of values for in and out of domain. The boolean way seems to be the more common way of doing this in NCL and numpy, and will work with masking.

Start by reading the existing wrf_user_ll_to_ij and ll_to_xy routines to understand their coordinate conversion and domain handling. Define the behavior for points inside and outside the WRF domain, including the return shape, then verify that both cases are represented by the resulting check.