Start to get rid of storage examples in MSG

Start to get rid of storage examples in MSG

Convert this test to a s4u-c example based on disks. It matches the
c++ version.
xbt_dynar have been replaced by fonctions that allocate and build
arrays of objects that the user will have to free.

Problem: there is no convenient xbt-free way to handle dictionnaries
yet, hence a xbt_dict remains to access properties.