add FindEigen3.cmake copied from Eigen/cmake
[TemplatePoint] Added DIM as nontype template parameter (default=3).

Changed also
- the data type that holds the coordinates from pure C array to std::array<T, DIM>,
- the implementation of write() and read().
Added weighted point class and a test.
Renamed file for the test WeightedPoint classes.
Changed one constructor of TemplatePoint.
Changed argument from initializer list to std::array to make old compilers happy.
Use std::copy instead for for-loops, std::array iface for c-array access.
Improve TemplateWeightedPoint constness.
[TMeL] Correct test argument order.
add typical install paths of eigen3 on *nix in FindEigen3.cmake
add CMake option OGS_USE_EIGEN
configure Eigen in travis
Merge pull request #219 from norihiro-w/feature-Eigen

Use Eigen in OGS
Merge pull request #215 from TomFischer/GeneralizeTemplatePoint

Generalizes TemplatePoint and adds class template TemplateWeightedPoint
[MaL] Update integration tests.
[MaL] Replace Gauss-Legendre algorithm with compile-time implementation.

Any weighted sum of type sum(f(x_i)*w_i) can be computed at compile-time
provided constexpr input. This is implemented in Integration.h file in
WeightedSum<Method>::add(f).

The integration method must provide values for x_i and w_i corresponding
to its order. An example implementation is the Gauss-Legendre integration
method up to fourth order.

Even if constexpr is not available in every compiler some of them are able
to perform same optimizations.
[MaL] Rename Integration.h to WeightedSum.h.
Find Eigen on Macs configured with Boxen.
Download Eigen into the build dir if it is not found on the system.
Merge pull request #211 from endJunction/WeightedSumGaussLegendre

[MaL] Replace Gauss-Legendre algorithm with compile-time implementation.