Fix the possible issues in cotangent weighting scheme

[erdenbatuhan]

• The cotangent weighting scheme is probably not working as the deformation seems smoother with constant weights.
• Rewrite the function in a cleaner way.

interactive-arap/src/Arap.cpp

Line 61 in b864ab7

m_weightMatrix = Eigen::MatrixXd::Zero(m_undeformedVertices.rows(), m_undeformedVertices.rows());