Replaces all NetCDF related packages by xarray

[senesis]

I hope using xarray could be OK for all CLiMAF stakeholders

Clean-up of nectdf libraries use :

• xarray becomes the sole package used;
• module anynetcdf is discarded and
• only functions provided by module nectdfbasics are used throughout CliMAF code, except for a very few calls to xarray (in driver.cread and classes.cdatset.check)

Test suite run is OK on ciclad, except for two plots which invoke accessing data at time=3000., and which don't succeed with version 2.0.2 either

Bu the way : the total list of packages import in CliMAF whcih are not provided by the standard library would now be :

• IPython.display.Image : fucntions.py : iplot(), implot()
• natsort : classes.py, for ensembles
• numpy : driver.py, functions.py
• xarray : netcdfbasics, classes, driver, functions
• yaml : ESMVaTool_diags

[rigoudyg]

The changes are integrated in pull request #240