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 the option HESSIAN from the code.

Workflow file now includes a header.

Update: Merge test