Get methods of pyphare.Run should not specify the file where py_attrs are retrieved from

[rochSmets]

In the methods GetDomainSize and GetDl of pyphare/run.py, the file where the attributes are is specified. Hence, These methods won't work if the file does not exist. A better strategy should be to list all the h5 files of the run, and get one (whatever, they all contain the same attrs) to deserialize the needed quantity (that is cell_width and domain_box)