Decouple the LibXSPEC_jll from SpectralFitting and make it available as
an additional package kept in this source tree. This package is called
`XSPECModels`, and loading both SpectralFitting and XSPECModels will
give exactly the same functionality as before.

SpectralFitting can now be installed without the (currently only)
Linux/Macos precompiled binaries.

Updated the model paths to use `~/.julia/spectral_fitting_data` instead
of the LibXSPEC_jll artifact directory, since this might be in an
unknown location. The model data can now also be used between version
changes without problem and more easily inspected / modified.