Removed unused variable src_type in postproc

 - in routine convolve_with_stf
Hiding seistype option from post_processing

 - There was no real support for velocity seismograms anyway.
Removed seistype from SOLVER

 - See last commit, removed variable seistype from the param_postprocessing written by parameters.F90
Updated STF section inparam_advanced

 - Make errorf default for "earthquake-like" runs
Explanation of STFs in manual