[wip/sot-core-v3] Release v3.1.9

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
Fix some compilation warning
Make Multiply_of_* a VariadicOp instead of BinaryOp