Workshops should work with multiple durations

[reid-a]

SC21 BoF feedback indicated a slight preference for half-day workshops, with nontrivial preferences for full day and two day workshops. Related to issue #90 and issue #91. See the SC21 analytics for source info.

It's not obvious how actionable this is, but it definitely informs our plans for the future, organizers want flexibility.

[reid-a]

Related to issue #95 -- if lessons are "stackable", then multiple durations can be achieved by building larger or smaller "stacks", with correspondingly larger or smaller coverage.

It's also possible to build separate lessons which cover nominally the same material at different intensities. This is a heavier lift, of course.a lot more work.

[reid-a]

Retitled this issue to be about workshop duration rather than lesson duration, to better reflect the actual BoF question, and to discourage the interpretation that we should have "fast" and "slow" lessons for the same material.