ENH: Add keyword add_lines in method plot_grid in gridmapdisplay

[jfigui]

On occasion you would like to have the grid lines but not the coast maps in the plot. Separating the two would be useful (see https://github.com/MeteoSwiss/pyart/blob/dev/pyart/graph/gridmapdisplay.py)

[mgrover1]

I think this is also related to #1229 ...

[mgrover1]

Would you be interested in submitting a PR similar to the enhancement you added to the meteoswiss fork? @jfigui

[jfigui]

@mgrover1 ,
Yes, I can do that. I will try to find a slot to do it in the next two weeks. Pin me again if you see that nothing has happened by then :)