Add a few methods in Matrix3fX and Vec3fX:

  Matrix3fX::isIdentity,
  Vec3fX::norm, Vec3fX::squaredNorm,
  Vec3fX::setZero,
  operator * (double, Vec3fX).x
Merge pull request #27 from florent-lamiraux/master

Add a few methods in Matrix3fX and Vec3fX: