provide option to only run a configurable number of iterations of Scenarios

[jeremyandrews]

• introduce --iterations config option (which auto-enables --no-reset-metrics as otherwise the final metrics can be very confusing/irrelevant; when using --iterations the metrics should confirm that all Scenarios and Transactions were run as intended)
• track iterations in GooseUser
• exit cleanly if iterations is set and GooseUser executes the scenario this many times o this required adding a new channel where GooseUsers can report to the Parent they are shutting down
• introduce scenario metrics, disabled with --no-scenario-metrics (or GooseDefault::NoScenarioMetrics) o this was required to provide visibility into how the load test is configured, ie when --iterations 1 is specified
• introduce scenario log, enabled with --scenario-log FILE-NAME (or GooseDefault::ScenarioLog) and formatted with --scenario--format FORMAT (or GooseDefault::ScenarioFormat) o all metrics can be logged, so we add this log type too
• add tests o stand-alone and gaggle tests, with different schedulers o found/fixed a few gaggle bugs introduced when adding TestPlan support
• update documentation o added new page for Scenario Logs o updated the Metrics page so example includes Scenario Metrics o general cleanups/updates to cross-reference Scenarios (and Transactions, and Requests) o started switching from .png images to .jpg images in the book for better compression, due to the limit of Cargo package sizes on crates.io
• closes https://github.com/tag1consulting/goose/issues/467

A new SCENARIO METRICS table makes it clear that --iterations was used (in this case --iterations 2):

 === PER SCENARIO METRICS ===
 ------------------------------------------------------------------------------
 Name                     |  # users |  # times run | scenarios/s | iterations
 ------------------------------------------------------------------------------
 1: AnonBrowsingUser      |        9 |           18 |        2.25 |       2.00
 2: AuthBrowsingUser      |        3 |            6 |        0.75 |       2.00
 -------------------------+----------+--------------+-------------+------------
 Aggregated               |       12 |           24 |        3.00 |       2.00
 ------------------------------------------------------------------------------
 Name                     |    Avg (ms) |        Min |         Max |     Median
 ------------------------------------------------------------------------------
   1: AnonBrowsingUser    |        1056 |        939 |       1,189 |        939
   2: AuthBrowsingUser    |        1565 |      1,433 |       1,628 |      1,433
 -------------------------+-------------+------------+-------------+-----------
 Aggregated               |        1183 |        939 |       1,628 |        939

This new table also includes a graph in the --report-file: