update TaylorGreen example, replace dtAdvecCrit method_key by dtCrit, add default filename in dt_adapt operator

Add baroclinic operator and associated example for RTI benchmark (which is still not working properly) + some documentation + several addings/improvements  

Adaptative vorticity projection according a given criterion + velocity correction (to prescribe proper mean flow and ensure the desired inlet flowrate) + some updates in adaptative time step procedure

improvement of the adaptative time step operator (the choice of dt_advection criterion now relies on the use of a method_key in a dictionnary)

general updates with in particular, simulation with adaptative time step, script for flow past bluff bodies, forces computation

Update differential_operations and stretching (fix bug + optimizations) - Add GlobalTests (test functionnalities, perf ...)

fix some bugs in stretching operator/differential operator + add finite differences validation for TG benchmark + add some problems in Examples