Add condition operation and forgetting

[khosravipasha]

1. Condition might be slightly different than what we have implemented. Maybe rename to something like conjoin_literals.

2. In what cases this is possible? I get error when trying it on Sdds.

[guyvdbroeck]

The naming issue was resolved in e92c417 where condition is now conjoin in the transformations.
We should still consider adding a condition operation though, and a forgetting one -- leaving this issue open.

[khosravipasha]

What was the main difference with condition vs conjoin? Would be a nice example in the docs.
We already have forgetting, but maybe not for all types of circuit.

[guyvdbroeck]

The function after conditioning on X no longer depends on X. The function after conjoining with X still depends on it and fixes it to one value.

[khosravipasha]

renamed since original issue was resolved and other new things were suggested.