List of things to do in the package

* Add the v- and w- transformations to extendbba

* Add other rules (mean, w-parametric, conjunctive, disjunctive)

* Add computation of distances btw bbas

* Go to multivariate modelling (extension through balooning, marginalisation, ...).

* Add Dempster's rule of conditioning (for the moment, done through dempster aggregation)

* Rework reordering - merge the ascending/descending parts and factorize comparisons of elements as external comparative functions (so that comparison mode become a parameter). 

* Extend the package to model belief functions on real-valued intervals. 