Sparse vector
=================
The :cpp:type:`SpVec` class for sparse vectors. It is an alias for
:cpp:type:`Eigen::SparseVector\<double, Eigen::RowMajor>`.
Its purpose is to provide convenient mechanisms for performing basic sparse vector
operations. All of the member functions implemented in :cpp:type:`Eigen::SparseVector\<double, Eigen::RowMajor>` can be directly used by :cpp:type:`SpVec`.