Issue184 add ipsl keywords and vertical level

Adding the extra keywords coming with the metrics in the form of a new compute_metrics_ExtInfo function.
Need to also update the __init__.py in the tmp/.../pcmdi directory.
Coming with this, a couple of modifications have been done in the driver.
Also, I modified the driver so it can search for a 2D variable on the pressure levels rather than in a 3D variable.
Finally, a new parameter file comes with it, with the added parameters:
- attributes_provided # do you want the standard amount of info, or do you the IPSL keywords?
- extract_from_3DVariables # Do you want to extract the variables on the pressure levels from a 3D variable or do you want to search for a 2D variable?