Dump counters are no global variables anymore

 - iseismo, isnap and istrain are now local variables set in sf_time_loop_newmark and symplectic_time_loop and passed down into the dump routines. That should make everything a bit easier to debug.
 - Removed several VERY outdated routines used to dump the wavefield into ASCII files. They would probably not have worked anymore anyway.
Dump timestep zero as well

 - True for all three dump types (seismograms, xdmf/vtk and kernel dumps)
Merge branch 'master' of https://github.com/geodynamics/axisem
Time variable in postproc starts at 0
Merge branch 'correction_time_zero'
Blocked unnecessary netcdf dump in first time step
Bugfix: Number of seismogram dumps was too small

 - Led to subtle boundary violation in NetCDF output, where first sample of seismograms was overwritten by value of the last from another station
Tidying up
round to nearest integer instead of down. Do to floating point inaccuracy, this led to

wrong numbers before