Integration of custom VTK edition

This is open for discussion.

This integrates a [custom VTK/ParaView subset](https://github.com/ufz/catalyst-io.git) currently containing `vtkXMLIO` and all its dependencies as well as the foundation for [in-situ visualization](http://catalyst.paraview.org) (ParaView's client/server implementation) as a CMake ExternalProject if it is not installed system-wide.

The compilation takes under 5 mins (on 2 cores) for me.

For `OGS_BUILD_GUI=ON` this is not used because I haven't created a edition which includes all functionality for the GUI.

We also have to increase the required CMake version to 3.0.0.

Things to fix:

- [ ] Builds only on one core on Windows
- [ ] Install CMake 3.0.0 on Travis

This is related to #213, #55, #196.