Change all instances of taking argument "const Eigen::VectorXd &" to "const ConstVectorRef&"

[ManifoldFR]

This also the default constructor of the MPC class.

Merge this after #26

[ManifoldFR]

This change is part of the following stack:

• Change all instances of taking argument "const Eigen::VectorXd &" to "const ConstVectorRef&" #27 ◀
  • Add pixi support and CI #28

_{Change managed by git-spice.}