Integrate forward_TV_mex.cpp in build system about gerardus HOT 4 CLOSED

No urgency here - there is a Matlab version that does the same thing, just a 
little bit slower. If the images are quite small, the Matlab version might even 
be faster.

No worries, Darryl, it has Priority-Low. It's more of a note to myself so I 
don't forget. But I'd like to see whether the function would work with the 
current build system. It doesn't link to external libraries, so it may be quite 
straightforward ;)

Darryl, I have added it in revision 1673: 
[https://code.google.com/p/gerardus/source/browse/trunk/matlab/FiltersToolbox/CM
akeLists.txt?r=1673#57 CMakeLists.txt]

If you can, please check that it works for you by building Gerardus. Or you can 
build only your auxiliary file doing

$ make forward_TV_mex

The name of the function should change, though. For an auxiliary function, 
forward_TV_aux.cpp would be less confusing than forward_TV_mex.cpp, I think ;)

PS. The build gives a warning, in case that's of any interest:

[100%] Building CXX object 
matlab/FiltersToolbox/CMakeFiles/forward_TV_mex.dir/forward_TV_mex.cpp.o
/home/rcasero/Software/gerardus/matlab/FiltersToolbox/forward_TV_mex.cpp: In 
function ‘void mexFunction(int, mxArray**, int, const mxArray**)’:
/home/rcasero/Software/gerardus/matlab/FiltersToolbox/forward_TV_mex.cpp:123:35:
 warning: comparison between signed and unsigned integer expressions 
[-Wsign-compare]
Linking CXX shared library forward_TV_mex.mexa64
[100%] Built target forward_TV_mex