fixed an important bug in the call to compute_pressure_whole_medium()

potential_acoustic was used instead of potential_dot_dot_acoustic when dumping the pressure field to disk (but JPEG pictures were fine).
added a test to check that the CPML flags read from an external file make sense, and call a stop statement otherwise.
Also improved a few print statements and a few comments.
Merge pull request #39 from komatits/devel

added a test to check that the CPML flags read from an external file mak...
added an option (in setup/constants.h.in for now) to display the PMLs in a different color in Postscript vector plots.
This option could/should be moved to DATA/Par_file one day.
Merge pull request #40 from komatits/devel

added an option (in setup/constants.h.in for now) to display the PMLs in...
avoid an edge effect if the PML color number is too high for the display
Merge pull request #41 from komatits/devel

avoid an edge effect if the PML color number is too high for the display
improved a comment
Merge pull request #42 from komatits/devel

improved a comment
removed three unused routines (Alexis Bottero had noticed that they were unused)
Merge pull request #43 from komatits/devel

removed three unused routines (Alexis Bottero had noticed that they were...
avoided a potential side effect for array "rmemory_fsb_displ_elastic" owing to some "if" statements: the array was sometimes unallocated
Merge pull request #44 from komatits/devel

avoided a potential side effect for array "rmemory_fsb_displ_elastic" ow...
updated and cleaned some comments
Merge pull request #45 from komatits/devel

updated and cleaned some comments
done porting periodic boundary conditions to MPI
Merge pull request #46 from komatits/devel

done porting periodic boundary conditions to MPI