NXtomo: improve robustness and readibility of node name vs path handling

• remove some 'mixture' between NXobject name and data_path parameter used by the NXobject.save function
• data_path default parameter is now None instead of an empty string
• if None provided then data_path becomes the NXobject name
• if the NXobject as no parent then 'path' property return 'root'
• improve doc to avoid providing 'node_name' for NXtomo (as they are expected to be high level object - no parent expected for now)

close #149 (closed)