Introduce cycle numbers earlier

[klieret]

Questions about cycle numbers already appeared in “Basic file I/O with Uproot” (potentially because of double-execution of cells)

[jpivarski]

These should be dispatched pretty quickly, e.g. with a parenthetical phrase. The cycle numbers have to be there, but they're not important. If too much attention is drawn to them, it would derail the lesson.

[stale]

This issue or pull request has been automatically marked as stale because it has not had recent activity. Please manually close it, if it is no longer relevant, or ask for help or support to help getting it unstuck. Let me bring this to the attention of @klieret @wdconinc @michmx for now.

[stale]

This issue or pull request has been automatically marked as stale because it has not had recent activity. Please manually close it, if it is no longer relevant, or ask for help or support to help getting it unstuck. Let me bring this to the attention of @klieret @wdconinc @michmx for now.