pass slicing argument to od.plot..._section()

[Mikejmnez]

Something like

ax = od.plot.horizontal_section(varName='Temp', ..., xstep= 2, ystep=5)

where the xstep and ystep arguments are used as follow:

da = da.isel(Xdim=slice(0, Nx, xstep), Ydim=slice(0, Nx, ystep))

within oceanspy.plot.py

Something similar would occur when making vertical sections (with slicing along the horizontal dimension)... This would greatly speed up making horizontal and vertical section plots (and animations) of very large datasets like the LLC4320.

[Mikejmnez]

closed by #377