Test cases for quantum channel

[Naphann]

In order to be confident that QuISP is performing correctly when the code base changes.

We need a way to test the quantum channel. Either by simulation or unit tests.

For simulation we need

1. Sharing link-level base Bell pair without memory error and BSA efficiency at 100%
2. Performing tomography without purification
3. Check if the density matrix output give close enough result when measurement is done enough times (maybe consult Oka's thesis on this).

The test should include

1. single errors (only one of the possible errors turned on)
2. turn all errors on but stay mostly biased.
3. test loss independently to Pauli errors.
4. Test whether loss affect the Pauli errors.