Should numba be a mandatory dependancy?

(I open this issue as reminder to be discussed later)

Currently, numba is not a dependency of lazylinop package. If we want to keep numba optional, we must ensure that everything works without numba: define some fake njit/prange/… functions (probably as a fake_numba module that could be imported in various parts of lazylinop which use numba) and add a variant of our tests where numba is not available.

We could also decide to make numba a mandatory dependency, as it is essential to the performance of some lazylinop functions