Hmmm looking at previous CS definitions great care seems to have been used to ensure that float casting is performed. That's why I suggested it in this PR. So either it really doesn't matter because PROJ.4 deals with it appropriately, or the Cartopy `Geostationary` class should default to float `satellite_height`,`false_easting` and `false_northing`. 

I notice that the Cartopy `TransverseMercator` has default float `false_easting` and `false_northing` ... does this all really matter @pelson or @rhattersley ?