Added EigenSparseUtil class for ease of coverting SGSparseMatrix to Eigen3 SparseMatrix

- Added a template class EigenSparseUtil in shogun/mathematics/eigen3.h, defined in shogun/mathematics/eigen3.cpp inside shogun namespace. 
- Also, defined Triplet struct for eigen<3.0.93 within Eigen namespace in shogun/mathematics/eigen3.h, so that we can avoid writing version specific code each time we use it. 
- Rewritten the log_det method for sparse matrices in CStatistics using EigenSparseUtil::toEigenSparse(..) to make it work with the new addition. Tested with both eigen<3.0.93 and eigen>3.0.93, unit tests working.