model: new agent to compare real life/simulation

We add a new agent CompareDateAI to compare real life data
(date-semis-bourville) with simulation of the GenericCropModel.
model: add a Global.hpp file

Share functions, constants and enumerations in a Global.hpp header.
model: add tdev/udev observation for GenericModel
model: improve CompareModel debug
model: fix model computation with leap year

The fp vector is not initialized properly with classic and leap year.
Todo--
Todo++
Readme: switch to markdown