Time and timestamp conversion

[roaldarbol]

Following on from this conversation on zulip, I think it would be great to have methods to access both time (since start, in whichever unit, see #59).

The proposition is to:

1. Encode start_time as a metadata field (needs to have #19 implemented in some form)
2. start_time can be either a numeric/ difftime in which case it's the time since 0 that tracking should begin (enything before should be NA) or a datetime timestamp.
3. Have methods for converting between difftime (not the class, just time since start) and timestamp.
4. Ensure that further methods do not encounter issues with timestamps - such as calculate_ functions