updated Xspec method to call fortran for relagn

[krutkow]

Xspec has updated the interface for calling fortran. A use xsfortran line is now required in local models written in fortran.

Please see:
https://heasarc.gsfc.nasa.gov/docs/software/xspec/faq.html#fortran-local-model-change
and the release notes:
https://heasarc.gsfc.nasa.gov/docs/software/lheasoft/release_notes/RelNotes_6.36.0.html