Add a benchmark for plotting 2-D Cartesian xarray

[seisman]

Grid Increment	Matplotlib Plot (s)	Matplotlib Total (s)	PyGMT Plot (s)	PyGMT Total (s)
0.05	0.0107	0.0691	0.0067	0.1469
0.01	0.0083	0.1876	0.0477	0.4385
0.005	0.0124	0.2585	0.1441	0.9392