[wip/sot-core] Release v3.1.8

Add Event::setOnlyUp
Add entity VariadicOp and update Mix_of_vector entity.
Make Switch derive from VariadicAbstract
Enhance VectorSelecter and MatrixComparison
Add command Sot::getSize
Fix usage of activated DoF in entity Sot
Make Multiply_of_* a VariadicOp instead of BinaryOp
Clean code of operator PoseUThetaToMatrixHomo, MatrixHomoToPoseUTheta…