added unnested structere for Vadere Seed Manager

• VadereSeedManager usually creates a structure of {'vadere': {'fixedSeed': ...}, 'omnet': {'fixedSeed': ...}} for the fixedSeed
• this structure does not work when using vadere only
• therefore, I created the variable is_seed_nested. If False the structure is just {'fixedSeed': ..., 'param1': ..., 'param2': ..., ....}
• the default is True, i.e. existing code is working as usual