Extract CellSet to ViennaCS Library

[exilief]

This is the implementation for #77.

I removed all the cellSet files except csSurfaceCellSet.hpp (which wasn't moved to ViennaCS yet) and adjusted the includes.
(I accidentally modified psSmartPointer in csSurfaceCellSet.hpp, should I undo those changes?)

What still needs to be changed is all the CMake stuff and Python bindings, and the Readme (and testing if it works).

If the LS and Ray dependencies are replaced with only ViennaCS, it might be a problem, because in tests/CMakeLists.txt and examples/CMakeLists.txt, the VIENNAPS_SYSTEM_VIENNALS and VIENNAPS_SYSTEM_VIENNARAY variables are still used. Should it still do the find_package(ViennaLS) and other calls, even if it's no longer a direct dependency? Or can all 3 libraries be listed as a dependency?
I don't want to mess up the build system, so I'd prefer if you do this part or tell me how to proceed.

[exilief]

I now experimented with updating the build system. In target_link_libraries() in CMakeLists.txt I replaced ViennaLS and ViennaRay with ViennaCS; the other mentions of LS and Ray I left in. This appears to work, but please check if this doesn't break anything else.

If this is okay, then I think only the Python bindings are left to do. In python/pyWrap.cpp the old cellSet location is still used, but I don't know how to deal with this.

[tobre1]

Please change the target branch for the merge to viennacore.

[exilief]

I changed it. Do I need to rebase/merge the conflicts, or can you do that?
Since I already removed the cellSet files, your changes to them will be lost, so should we maybe revert that commit?