making plotting WRF-specific

[jonathancallahan]

This isn't a precisely defined task but I feel the various plot_~() and layer_~() functions (or some new ones) need some changes to make them more user friendly in the context of working with WRF output.

Here are some specific suggestions for the layer_~() functions:

• [x] layer_vectorField() accepts separate uRaster and vRaster rasterLayers as arguments and assumes they are on the same grid. Instead, I think it should accept a raster argument (which will be a RasterBrick object) and uName and vName arguments and create the internal uvRasterBrick from that information.
• [x] To work in a similar manner, layer_raster() should also accept arasterandvarNamearguments. As a nicety, it could also handlerasterbeing a single variable _rasterLayer_ object in which casevarNamewould not be needed (default ofNULL` could be ignored).

And I'm not thrilled about a new "class" of objects implied by the plot_~() functions. Instead, I think these should be functions/methods associated with the somewhat abstract wrf_ objects -- basically just a RasterBrick full of WRF output variables.

Some suggestions for the plot_~() functions (now renamed as wrf_~() functions:

• [x] wrf_basePlot() -- No change except maybe have title default to "WRF output for hour on ". Can we use raster@title to help create this?
• [x] wrf_rasterPlot() -- Like the functions above, this should accept raster and varName arguments.
• [x] wrf_standardPlot() -- Like the functions above, this should accept raster argument and the names of other parameters for background, U and V.

[tabrasel]

wrf_basePlot() is always called by higher-level functions like wrf_rasterPlot() or wrf_standardPlot(), which are actually given the rasters from which we can extract their titles. So I think it is better to set the title of the plot within the higher-level function and just passing that string down to wrf_basePlot().