Rethink `get_namespace` helper

[Saransh-cpp]

There are situations where that isn't uncommon. For example, you could have fixed numerical coefficients that aren't going to be autodiff'd: evaluate_polynomial(variables_jax, coefficients_numpy).

In our case, and what made me wary of this change, is that we are going to have to load e.g. a survey footprint from file, which is going to be a fixed numpy array. Then we are going to pass this into the simulation, which will now potentially require a conversion to a jax array for no reason other than making this helper happy.

Originally posted by @ntessore in https://github.com/glass-dev/glass/issues/604#issuecomment-2800896862