Updated SH viscoelastic modelling to the new grid.

Now it is possible to specify WOLFE_C1_SL and WOLFE_C2_SL in the JSON input file.
Documentation is updated as well.
Smaller output improvements.

Fixed a bug which prevented to stop at max FC when FC_END was not a multiple of FC_INCR.
Additionally L-BFGS will try step length 1 first every time a new frequency stage is choosen.

Added the option WRITE_FILTERED_DATA to write time filtered measured data to disk.
If WRITE_FILTERED_DATA is not set in the JSON file, it will be set to 0.

Added debugging output for VP and renamed some variables to be consistent within the code.

Removed all checkpointing things in the code.

Removed TIME_FILT==1 from SH toy example

Removed the option HESSIAN from the code.

Workflow file now includes a header.

Updated documentation to include all new variables which are available in the JSON input file.

Update: Merge test

-Remove src_relicts
-Clean up of the Makefile in src/

Removed gfortran and f2c as known dependency.

adjustment of acoustic toy-example, scaling of the elastic explosive source and acoustic free surface condition.

Code in contrib/aff/tests is not required as part of Seitosh.
However, the test code requires additional settings like proper interlanguage
linking (C++ and Fortran). To reduce the constraints on the installation
environment, discard the subdirectory tests in contrib/aff entirely, as well
as all references to the test code made in the Makefile.

Programs in contrib/stfinv/tests are not required by DENISE but may cause
confusion since they require libtfxx which is not part of the vendor import.