pygr: overhaul autoscale on y axis

Use selected region (window) and extent it
if the last added point has not been not inside.
pygr: fix grid line calculation

* Calculate xorg, yorg for gr.grid function.

Fixes: #1313 https://trac.frm2.tum.de/redmine/issues/1313
pygr: Change default background color for plot canvas.

Use white background color as default:
* no interference with other colors, e.g. grid lines or plot curves.
pygr: new formula for xorg, yorg grid calculation
Fixed 'clean' target (on system w\out Sphinx)