Use ns3::Time for time values in scenariofile

[ltoenning]

Related to #55.

Proposed Changes

Use the parsing capability of ns3::Time to read time values with a unit from the scenariofile like "5s".

Updated documentation for default_delay and stop_time will follow when switching completely to the new scenariofile parser.

Definition of Done (optional)

• Feature is implemented
• No known bugs that stops the correct execution
• CI / CD pipeline passed
• The code guidelines were followed
• [ ] Unit and / or Integration tests for the new feature
• New feature was added to the documentation

[janagoe]

Nice workaround while we are not able to use the unit libraries yet. However, I would like to use seconds as much as possible in the scenario files. Milliseconds becomes rather unreadable with many zeros.