elastic-tube-1d fortran-module: precice.f90 mixed with the case files

[MakisH]

After merging #607, I noticed one small thing that I would have done differently and slipped my review: the precice.f90 is downloaded into src/. I would normally put it under thirdparty/ (or even get the repository as a git module), to keep it clearly separated. We should also add it to the .gitignore.

@YonatanGM If you want to contribute this as well, that would be great, but I can also do it later.

Originally posted by @MakisH in #607 (comment)

[YonatanGM]

Will have a look and get back to you in a bit @MakisH

[YonatanGM]

I tried adding it as a submodule in #614