Updated documentation: added new SOURCE_TOPO and REC_TOPO options; removed references to anisotropic acoustic wave equations

Implemented topography option for sources and/or receivers which, if used, are automatically placed a certain depth underneath the topography, assumed to be the first interface from top in a given model.

Revert change of source.dat from commit 18b6cae6 which moved the source position; this modification broke ALL available tests.

Bug fix on PML boundary. Tests differ from reference results - fix needed. 4th order time doesn't work - fix needed

Update of compare script to output min/max values of test result, reference result and difference on terminal.

Update of test scripts to allow running with valgrind memory checker, or to allow specifying specific program executables.

Update of Makefile: removed duplicate entry; added check for local source code changes via git command

WEQ_AC_TTI implemented but wavefields do not show expected anisotropy.
Unsure about correctness of the stress-strain relation.
PML needs to be fixed for all wave equations.

PML is broken for all wave equations. Reflections at the right and bottom edges occur.

 implementation of acoustic isotropic wave equation (WEG=AC_ISO). Absorbing boundary conditions not yet finished.

Merge of own changes with latest commit in branch 'develop' of https://git.scc.kit.edu/GPIAG-Software/SOFI2D.

Calculation of variables fipjp, f and g moved to prepare_update_s.c vor elastic case. Calculation of same variables for viscous case moved to prepare_update_s_visc.c.

Updated harmonic averaging to handle situation where negative values are encountered and infinity could occur.

Allow for arbitrary snapshot grid increments while retaining correct output slices; fixed handling of number of snapshots which differed between SOFI and SNAPMERGE; fixed SNAPMERGE to handle multiple shots if present; updated reference snapshot results of examples as they now have one additional slice which has been ignored before.

Bugfix in initsrc if RUN_MULTIPLE_SHOTS=1; led to crash as buffers weren't free'ed correctly and a domain without source was trying to write a signature; also fixed SU header sx/gx of signature output file.

Updated code to assume external model is starting at x=0,y=0 rather than x=dh,y=dh which was assumed until now; the change concerns primarily the calculation of indices from src/rec coordinates and vive versa; src/rec position files for the example have been adjusted such that no new reference results are required (shift of src/rec coordinates by dh).

Make parameter TAU optional and remove from examples; only used when simple models are created internally.

Removed NP from globvar struct (only req. in one place); added log of Cartesian MPI grid; added GGRID variable in globvar struct to store which global grid points each processor handles in preparation for subdomains of unequal size.

Restructuring of main code. Moved readmod, tensor component preparation and time loop to separate functions and files.

Fixed determination of min/max phase velocity for TTI case and moved all calculations from checkfd to readmod routines; consequently, call to checkfd in main program now the same for all wave equations; min/max velocities now stored in globvar struct.