Simplify comparisons

[kinow]

This PR replaces var == True by a simple var test, and also uses is None instead of == None (see ref)

[davidhassell]

Hi,

I won't merge this, not because I disagree with it (on the contrary), but because it highlighted a slight change in the functionality that was needed in cf.functions._section, and it was easy to implement all of the other is None changes by hand, as there weren't many of them.

Many thanks

[kinow]

Thanks @davidhassell ! Some tests were failing in my environment (before the change too), so I couldn't confirm what exactly would be impacted by this.