New method `fidelity` for `Statevec`

[matulni]

This issue qualifies for UnitaryDESIGN 2026!

Context

Given two pure quantum states $\psi_1$, $\psi_2$ the fidelity is defined as

$$F(\psi_1, \psi_2) = |\langle \psi_1 | \psi_2 \rangle|^2.$$

Task

1. Add a method to the class graphix.sim.statevec.Statevec which allows to compute the fidelity of two quantum states.

Hint: Check :func: graphix.sim.statevec.Statevec.evolve_single.

2. Add a method isclose which evaluates if two quantum states are equal up to a phase. This method should compute the fidelity and check if it's equal to 1 within certain relative and absolute tolerance.

Hint: Check math.isclose.

3. Unit tests in graphix often resort to a simulation of two equivalent patterns (obtained with different methods), and the assertion that they represent the same unitary.
   Use the method implemented in 2) to replace

   assert np.abs(np.dot(state.flatten().conjugate(), state2.flatten())) == pytest.approx(1)

   and similar statements in the test suite.

Note: New methods should have the appropriate docstrings and unit tests.