Drop current version of Seitosh code. The contents are taken vom branch master
of [Seitosh](https://git.scc.kit.edu/Seitosh/Seitosh). The current master
commit in Seitosh is 890d2fe565.

With this vendor drop the new taper feature as implemented in libstfinv
becomes available. Source time function correction filters produced in the
Fourier domain can be tapered in the time domain prior to application.

Commands executed in the root directory of IFOS2D:

    rm -rfv contrib
    mkdir -pv contrib
    (cd ~/git/Seitosh/src/libs/libaff; tar cvf - $(git ls-files)) \
      | ( cd ~/git/IFOS2D/contrib; mkdir aff; cd aff; tar xvfps -)
    (cd ~/git/Seitosh/src/libs/libfourier; tar cvf - $(git ls-files)) \
      | ( cd ~/git/IFOS2D/contrib; mkdir fourier; cd fourier; tar xvfps -)
    (cd ~/git/Seitosh/src/libs/libseife; tar cvf - $(git ls-files)) \
      | ( cd ~/git/IFOS2D/contrib; mkdir libcseife; cd libcseife; tar xvfps -)
    (cd ~/git/Seitosh/src/libs/libstfinv; tar cvf - $(git ls-files)) \
      | ( cd ~/git/IFOS2D/contrib; mkdir stfinv; cd stfinv; tar xvfps -)
    git add --force contrib

Since Seitosh on purpose does not provide export branches where source code
exports would be assembled in a hand-picked fashion, whole subdirectories from
Seitosh are simply copied. As a conseqeunce, the directory libcseife now also
contains Fortran code not really required by IFOS2D.