eigen_converters.h removed

Feature matrix getters added to DataSet and InnerRepresentation
InitEigenVector/Matrix replaced with standard Eigen Zero() method
DataSetToEigenMatrix calls replaced with direct access to feature matrix

All calls of functions from eigen_converters.h are removed,
some typedefs from it are moved to ltr/utility/typedefs.h

Change-Id: I542b95610ac0a931eb3646629988c697e13d8f39