Add SeisSol to package index

[ashirrwad]

Added the SeisSol package listed on Issue: fortran-lang/webpage#50 to the package index.

[milancurcic]

Looks good to me. While at first this looked like a C++ project with Fortran bindings, on closer look it seems to be a mixed C++ and Fortran project.

We've had some discussions in the past whether Fortran bindings to a foreign-language project would be appropriate for the index. While I think they are, I don't think we've reached a consensus about it.

[certik]

I also thought this is a C++ project with legacy Fortran, but it seems it has physics stuff in Fortran and it is still developed in Fortran (see e.g. SeisSol/SeisSol#536), so I would say this package fits into our index.

(If it was just Fortran wrappers, then I would say we should have a category for Fortran wrappers to other packages.)

[ivan-pi]

For the record, the developers have told me they would like to remove Fortran soon. See also the comment here: fortran-lang/webpage#50. If you look closer, they already have an issue to migrate the physics stuff from Fortran to C++: SeisSol/SeisSol#538.

[ashirrwad]

As the package was still a part of checklist on fortran-lang/webpage#50 and the library and its documentation showed references to Fortran usability I figured it was still up for addition. While I did stick to the guide the criterias for package addition are a bit ambiguous to me as I am new to the language. If the package doesn't meet the requirements I will add a PR to revert these changes as needed.