[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.
Use Eigen 3.1.4.
Merge pull request #223 from bilke/ExternalProjectEigen

Configure Eigen as an external project (if not installed)
Removed debug output.