Add implementation of RPNGrid.getXY()
Add implementation of RPNGrid.getXY()

This patch adds the implementation of RPNGrid.getXY(), which
is the lat/lon -> x/y counterpart of RPNGrid.getLatLon.  If
called with conforming lat/lon arguments, it returns the
corresponding (fractional) x/y coordinates; if called with
no arguments it returns a 2D x/y coordinate grid.

meteokid/python-rpn,meteokid/python-rpn,meteokid/python-rpn,meteokid/python-rpn