eigen: Use Eigen instead of Boost::uBLAS.
cmake: automagically downloading Eigen

and some tweaks and cleanup to 3rdparty CMake file
cmake: first search for system-wide Eigen3

only in case none found download sources
Merge pull request #6 from torbjoernk/feature/eigen-w-cmake

made CMake look for Eigen3
Merge branch 'pr/82/head' into development

* pr/82/head:
  cmake: first search for system-wide Eigen3
  cmake: automagically downloading Eigen
  eigen: Use Eigen instead of Boost::uBLAS.

Signed-off-by: Torbjörn Klatt <t.klatt@fz-juelich.de>