Lat/Lon to X/Y routines should be made to work with a DataArray argument

[bladwig1]

While working on #38, realized that there is no easy way to do lat/lon to x/y (and vice versa) with sliced DataArray variables. Currently, the WRF data file is used to extract the relevant information, and slices are not possible.

The DataArray variable needs to contain the lat/lon and projection information in order for this to work, so an error should be thrown if not (or fall back on the WRF data file). In interputils.py, to_xy_coords() needs modification, along with the _ll_to_xy() in interp.py.

The metadata handling for cross sections (and line interpolation) should also be updated to take advantage of this.