Add possibility to control when the state is approximated

[hillmich]

What's the problem this feature will solve?

Currently the approximation will run either when the DD grows too large or evenly interspersed between between blocks of operations in the circuit.

Describe the solution you'd like

When creating a circuit, often it contains building blocks at whose boundaries approximation make sense. It would be nice to be able to control when the approximation is applied.

Possible ways I can think of right now:

• Add an opaque approx <fidelity> operation in the circuit.
• Pass an extra list of positions when the approximations should be applied in the circuit.