Meaning of start_time in nexus filewriter

[HEnquist]

The implementation of #1645 led to a question about what the start_time in a nexus file means. The two candidates are when the macro started, or then the scan started running.
See this thread on the Nexus mailing list: https://lists.nexusformat.org/pipermail/nexus/2021/001170.html

The current implementation uses the macro start: https://github.com/sardana-org/sardana/blob/develop/src/sardana/macroserver/recorders/h5storage.py#L230

Do we change it to scan start, or leave it as macro start?