JIT compilation with numba

[oashour]

My first thought was to use @jitclass, which I did for the numerics module in a4e2e21. However, this approach would be a long and painful road since every single class would have to be a @jitclass since they are all used in Calculator, and thus every single method in the entire package will have to be @njit compatible. This is simply not practical (nor desired, nor necessary).

The best option is a heavy refactoring of the code so that I'm computing the binned rate as a (n_velocities, n_masses, n_modes, n_bins) array, i.e., all the Single[...]Reach classes need to be heavily reworks so that I'm not looping over them and calculating the rate for each (v_e, m_chi) pair individually. Those classes should do all the setup, then the entire rate calculation can be taken outside the classes into an njitted method that does nothing but array operations.