Add support for controls to PostselectionCircuit

[ohuettenhofer]

This pull request allows Postselections to be controlled, which means that they are only active if all the control qubits are 1, and otherwise nothing is filtered out/removed.
This is different from how the postselected qubits themselves behave, where states are filtered out if any of the qubits are in state 1.

This makes it simple to implement an add_controls function for PostselectionCircuit which behaves analogously to the corresponding QCircuit function.

I also changed how n_qubits is calculated for PostselectionCircuit to fix a problem when fragments have different numbers of qubits.