Remove optional numpy-based backend and require numba

[leouieda]

We still support numpy-based functions for building Jacobians and forward modelling in Spline and elsewhere. These are a bit of a pain to maintain and we have to keep testing with and without numba. The numba version is much faster, parallel, and more memory efficient. With the state of numba these days, it's not worth keeping it as an optional dependency.

We should remove the numpy-based functions and add numba as a required dependency. This can allow us to experience with numba functions for other performance critical parts of the project, like BlockReduce.