Feature/reference models

The first bunch of reference MDPs, to be used in release 0. The details of their calculation are explained in the adjoint readme file (I think we should keep it at least temporarily). An example script, to be used in CDS servers is also provided, as help and documentation. The whole calculation took place in CDS servers, via toolbox.

For the moment, the reference MDPs are stored as files. Imo, they should become attributes of the observatory class, once it is ready. Now we are good to go for the main scenario of MDP selection.