cf conventions "stere" projection in cfscene is buggy

[mraspaud]

from Ulrich: I guess there is a small bug in line: https://github.com/mraspaud/mpop/blob/pre-master/mpop/satout/cfscene.py#L432 "scale_factor_at_projection_origin": eval( proj_dict.get("x_0", "1.0") as x_0 is already the false northing.