Fixing issues with PercevalBackend and adding PermanentBackend

[mateuszkupper]

1. Provided the default permanent evaluation from core.path.Matrix as a backend PermanentBackend:

• allows to calculate: probabilities, amplitudes, a density matrix
• only allowed for pure LO circuits
• see docs/notebooks/bose_hubbard.ipynb for an example

2. Refactored and extended the PercevalBackend:

• the extension allows for calculation of amplitudes, probabilities and density matrices using a chosen perceval backend (e.g. SLOS, NaiveBackend)
• only working for diagrams which are states (if a diagram has "open" inputs, an external perceval state needs to be specified)
• the backend calculates a single prob/amp when provided with a closed diagram (dom/cod == 0)

3. Fixed evaluating sums of diagrams with the perceval backend.
4. Fixed some import issues from optyx import ... -> from optyx.core.channel import ... used in optyx.core.zw/zx.

[giodefelice]

LGTM