Compute bottom azimut

Usage

get_bottom_azimut(aspect, north2x)

aspect: double - Angle of slope bottom on the compass from the North, clockwise rotation (in degrees) northern aspect : 0, eastern aspect : 90, southern aspect : 180, western aspect : 270
north2x: double - Angle from North to x axis clockwise. (in degrees) Default correspond to a Y axis oriented toward the North.