Fix SparseVector::insertBack() with custom scalar types

It fixes this compiler error:

/home/tav/git/Sleipnir/build/_deps/eigen3-src/Eigen/src/SparseCore/SparseVector.h:143:19: error: cannot convert ‘int’ to ‘const Eigen::internal::CompressedStorage<boost::decimal::decimal64_t, int>::Scalar&’ {aka ‘const boost::decimal::decimal64_t&’}
  143 |     m_data.append(0, i);
      |                   ^
      |                   |
      |                   int

This change matches what SparseMatrix does: https://gitlab.com/libeigen/eigen/-/blob/master/Eigen/src/SparseCore/SparseMatrix.h#L430-L438