#1076 GlobalVariationalOperator: generalize the class so that each RefGeomElt may get its own LocalVariationalOperator.

The idea is that now the second template argument is not a LocalVariationalOperatorT, but a tuple which contains pair <GeometricEltEnum, LocalVariationalOperatorForGeometricEltEnum>.
As most operators (all present in HappyHeart so far, with possibly GradientBasedElasticityTensor apart) do not need different implementations for each RefGeomElt, the new generic class is named clumsily GlobalVariationalOperatorSeveralLocal and an alias has been defined to cover the most common case where same type is given for all geometries.

Many safeties remain to be added (the tuple is for instance a pair only for that purpose: a static assert will check all RefGeomElt are properly given and in the right order).