Design per-exposure logs

We need to track what exposures are taken, as well as some context and AIT measurements.

Currently there are two outputs: a per-exposure text line in the per-night FITS directory, and a better-maintained and complete SQL table row. Neither of these are quite right (the LOG.txt is ill-structured and not up-to-date, and the sql table merges exposure essentials and measured quantities and comments.

Discussions will continue.