spatial: remove OperatorMul and cleanup operators

OperatorMul does not seem to be needed. I'm not sure about why it
was introduced in the first place.

The only point I see is that Inertia could declare the generic
OperatorMul as a friend and hence let future specializations access
its private members. However, all its private members are accessible
through getters, so this does not seem necessary.

Maybe I missed the point, feel free to discuss.

I checked that this change has no performance impact.