Timezone handling across elements

[kabilar]

Manually moved from datajoint/datajoint-elements#63:

NWB exports require time information as UTC with timezone offset as ISO 8601 extended formatted strings (docs and original discussion).

Our current approach in the elements is to ask the user to provide datetime information in UTC and assume local time zone when importing. This introduces inaccuracies if the local offset has changed since time of recording (e.g., daylight savings), or the user performing the export is in a different location from data collection. Modalities will differ in how they record metadata, so we may also encounter issues combining our session UTC with modality-specific recording start times.

I propose (a) adding timezone offset to session.Session and (b) treating modality-specific timestamps as encoded with this offset. This will permit analyses within DataJoint to consider local time, but will require NWB export functions to remove the offset from all modality-specific tables.