added write_output_SU.f90
refactored write_seismograms
refactored write_seismograms
"Buildbot ran code beautifier."
refactored write_seismograms
moved data output code to subroutine
fixed legal header in write_output_SU
removed duplicate implicit none
moved Seismic Unix reader to subroutine
changed seismogram filename extensions, as discussed
refactored write_seismograms
fixed two comments
for consistency with 3D code, added save_adjoint_kernels.f90
added list_for_Zhinan_for_SPECFEM2D_PMLs.txt (items that need to be done for PMLs; we will remove that file when we are done)