Database descriptions

[GabrielMouttapa]

Now the database of observations must respect a specific format to be read by the database module, especially for the physical constants and forcings (they have to match exactly the convetions of the SCM model). A reflexion has to be done about the possible transformations (affine or more complex) that can be described in the yaml file that describes the database. The idea is to make the yaml file comprehensible by reading but also to have enough possibilities for the importation of the database.