Add a 'keyFloat' function to sort "Duration" column numerically

[gscelsi]

... instead of lexicographically. This simple change solves #449 and #413.

Note This merge would fix sorting of the default contents of the "duration" column. However, this merge would make sorting not work if users change formatting following instructions in the User's Guide Formatting the Duration Column section. In that case the current lexicographic sorting would be required. Therefore at this point I think the best solution would be to decline this pull request and to change the default formatting of the "duration" column to "%H:%M:%S.%f".