change ModelChain default orientation_strategy

[wholmgren]

I'd like to change the default ModelChain orientation_strategy from 'south_at_latitude_tilt' to None. I think it's a horrible idea for the default setting to overwrite a user specified system.surface_tilt and system.surface_azimuth. My bad.

I meant to do this when releasing 0.4 but never made an issue for it and forgot about it. The change should probably wait for 0.5.