Improving `units::Duration`

## Brief feature description

The `units::Duration` class could have same additional features and improvements.

## Detailed information

- [x] rename the creation methods like `milliseconds` to `fromMilliseconds`
- some getter methods should then be renamed, e.g. `milliSeconds` -> `milliseconds` or `toMilliseconds`
- no final decision, please ask before starting with this
- [ ] conversion to/from all `chrono` types
- [x] `operator+=`, `operator-=` and `operator*=` https://github.com/eclipse-iceoryx/iceoryx/pull/501#discussion_r564463012
- [ ] abstractions for `TimePoint` for monotonic and system clock https://github.com/eclipse-iceoryx/iceoryx/pull/501#discussion_r560061177
- needs to be discussed and maybe another issue should be used for this