Centralize all backend imports from a single `pulser.backends` module

This features adds a new pulser.backends module that should serve as the single source for all backends. Ideally, the user can now simple do eg

import pulser

qutip_backend = pulser.backends.QutipBackend(...)

If pulser_simulation is not installed, an error message will appear suggesting its installation.

AttributeError: 'QutipBackend' requires the 'pulser_simulation' package. To install it, run `pip install pulser_simulation`.

The same is true for the pulser_pasqal backends.

Imports like

from pulser.backends import QutipBackend, EmuTNBackend

also work, but in this case the error message is not informative if the appropriate extension packages are not installed.

pasqal-io / Pulser