ignore warning about missing source projection when GEOLOC option used

[mdsumner]

message:

Warning message:
In warp_in_memory_gdal_cpp(x, source_WKT = source_wkt, target_WKT = projection,  :
  no valid projection in source, specified output projection will have no effect
 (only the extent and dimension will be applied natively)
 use 'source_projection' to apply the correct details for this source

but, output is perfectly fine (this from a rotated pole cordex input)

vapour_warp_raster(tb, extent = ex, dimension = dm, projection = proj, 
                        transformation_options = "SRC_METHOD=GEOLOC_ARRAY", resample = "bilinear")

[mdsumner]

I think this is related to issues in #118 - if we could tell that we had geolocation arrays then messaging would be clear