addStatement should not be a public method

[dan-f]

The addStatement method on Formula objects adds a statement to a graph but doesn't index it. But since IndexedFormula inherits from Formula, it gets the method too. If you call addStatement instead of add on an indexed formula your querying gets broken since the indices never update.

I'd be in favor of removing the addStatement API or aliasing it to add. Since add is already a method on Formula, we can just modify that method to detect whether its arguments are a statement or separate s, p, o, and g objects.

This relates to #42, IMO

[dmitrizagidulin]

+1 to this.