I need this check because runxlrd fails for python 3.3
line 15XX, in update_cooked_mag_factors
    elif not (10 <= zoom <= 400):
TypeError: unorderable types: int() <= NoneType()

I have moved this logic to update_cooked_mag_factors(), which I think is a better "catch all" place for missing WINDOW2 records. I'll commit soon.