Add spectral model from XSPEC

[renecotyfanboy]

We will need to simulate models from xspec since it is hard to port them all to pure JAX. Build a special class of SpectralModel that are evaluated through external softwares and in particular xspec.

[alessandropeca]

This would be extremely helpful! Models like MYTORUS or UXCLUMPY are built on XSPEC tables, and it would be extremely helpful to be able to import them.