Add more backends

Add backends for local and remote computation:

- `EmuMpsBackend` (https://github.com/pasqal-io/emulators)
- `EmuTnBackend` and `EmuFreeBackend` (https://github.com/pasqal-io/Pulser/blob/develop/pulser-pasqal/pulser_pasqal/backends.py)
- `QPUBackend` (https://github.com/pasqal-io/Pulser/blob/develop/pulser-core/pulser/backend/qpu.py)

To be addressed after #13 