Concentration field added

Need to be tested and added as well in timestep_compressible
Merge branch 'master' into concentration
Python script for reading and displaying spectra
Magnetic spectra not displayed when empty
Timevar-reading script
Merge branch 'master' into concentration
Application of the new spectrum output system to the concentration
New spectrum system handled (+ conversion from the old one)
Script reading VTK files added
Header now in the right order after spectrum conversion
Computation of integral length scales added
Default problem updated
Merge branch 'concentration'

Conflicts:
	src/common.c
	src/common.h
	src/documentation.h
	src/mainloop.c
	src/output/output_spectrum.c
	src/output/output_timevar.c
	src/problem/default/gvars.h
	src/problem/default/snoopy.cfg
	src/snoopy.c
	src/snoopy.h
Merge branch 'python'