Faster hypergeometric functions on numpy arrays for `tofu.physics_tools.electrons.emission`

[Didou09]

See:

• ENH: special: hyp2f1(a, b, c, z) implementation for complex arguments (a, b) scipy/scipy#23450
• https://github.com/fedro4/specfunc

Alternatively, make the homemade version better, getting inspiration from mpmath implementation of hyp2f1

Better: use the new Rust implementation in https://pypi.org/project/cfsem/