Add Timestamp.get_time and improve Timestamp.seconds_since_unix_epoch

[JEnoch]

1. Add a Timestampl.get_time property returning the time part as a NTP64 (int).
2. Make Timestamp.seconds_since_unix_epoch rely on new uhlc 0.6.3 NTP64.as_secs_f64() (Add NTP64::as_secs_f64() atolab/uhlc-rs#13), and improve its doc.

[p-avital]

You can return plain values rather than results here, since the operations are infallible and switching to a result later will not be a breaking change since the errors are forwarded as exceptions