Remove strong link to armadillo using wrapper functions

[m-rest]

Use a wrapper and a type definition to externalize the use of Armadillo like:

#ifdef ARMADILLO_WRAPPE
template <typename PrecisionType>
using Matrix = arma::Mat<PrecisionType>;
...
#elif EIGEN_WRAPPER
template <typename PrecisionType>
using Matrix = eigen::Matrix<PrecisionType, eigen::Dynamic, eigen::Dynamic>;
...
#endif
} // namespace wdf

[m-rest]

http://en.cppreference.com/w/cpp/language/type_alias for more information

[maxprod2016]

If someone have better idea or more clean/powerful way to do a wrapper (and possibly avoid macro) let me know.