Messy conventions for gv.set_map_boundary and latitude sign for SouthPolarStereo projection

[senesis]

Describe the bug To get a south polar stereo map from lat = -40 to lat = -90, you must call gv.set_map_boundary(ax, [-180, 180], [ -40, 90]) which is conter-intuitive

To Reproduce

fig = plt.figure(figsize=(8.5,11))
projection = ccrs.SouthPolarStereo()
ax = plt.axes(projection=projection)
ax.add_feature(cfeature.LAND, color='silver',zorder=1)
gv.set_map_boundary(ax, [-180, 180], [ -40, 90])
plt.show();

Expected behavior Intuitive values for latitudes range should be gv.set_map_boundary(ax, [-180, 180], [ -90,-40]) but this produces a blank map

And, by the way, the pole latitiude could be implicit for Polar Stereo projections!

OS: see #210

Environment see #210