agenda and agendaupdate: add --details length redux

[michaelmhoffman]

Implements #791. Resubmission of accidentally closed pull request #817.

To eliminate the possibility of conflicts between length and end_date/end_time, this raises NotImplementedError if input is supplied with both of the fields.

In the future, the following would be better conflict resolution approaches:

1. allow both end fields and length field long as they are consistent in each row
2. conflict resolution by command-line option (e.g. prefer end fields or prefer length field)
3. conflict resolution interactively