Fix/remove vtk

[Cakem1x]

This PR makes VTK optional when finding packages for LVR2's dependencies.

This change should fix the build issues in naturerobots/mesh_tools#45 and naturerobots/mesh_navigation#63.
I managed to reproduce the build failure from mesh_tools on my local machine (after removing VTK). With the change in this PR, I can build it on my machine without VTK (I get only warnings, no error anymore).

--- stderr: hdf5_map_io
CMake Warning at /home/docker/ros_ws/install/lvr2/lib/cmake/lvr2/LVR2Config.cmake:73 (find_package):
  By not providing "FindVTK.cmake" in CMAKE_MODULE_PATH this project has
  asked CMake to find a package configuration file provided by "VTK", but
  CMake did not find one.

  Could not find a package configuration file provided by "VTK" with any of
  the following names:

    VTKConfig.cmake
    vtk-config.cmake

  Add the installation prefix of "VTK" to CMAKE_PREFIX_PATH or set "VTK_DIR"
  to a directory containing one of the above files.  If "VTK" provides a
  separate development package or SDK, be sure it has been installed.

[amock]

Looks good :)
Note for myself: Check if VTK has to be changed to required if certain optional targets are build. (e.g. lvr2_viewer)