Exercise more capabilities of Swim in the tutorial application

[brohitbrose]

• Introduce an exercise to the reader to update swim.tutorial.DataSource to write to more Agents than just /unit/master. Edit the README in server accordingly.

• Introduce an exercise to the reader to create a stats value lane in swim.tutorial.UnitAgent that is populated by changes to histogram. Maybe suggest something that follows this skeleton. Edit the README in server accordingly.

• Since the data entries in histogram are pretty boring, maybe enhance them with some measure of central tendency (e.g. mean/median) and some measure of spread (e.g. range; stdev/variance might be harder with streaming inputs but it seems doable?) Will leave this decision to @Crista2019 ; have fun with it!

• Include solutions to a new branch (or branches, if you prefer that).

• In the READMEs, ask how one might change the UI code to accommodate for these server-side changes.

[Crista2019]

addressed in the tutorial_with_exercises branch:

• Introduce an exercise to the reader to update swim.tutorial.DataSource to write to more Agents than just /unit/master...
• Introduce an exercise to the reader to create a stats value lane in swim.tutorial.UnitAgent that is populated by changes to histogram...

[Crista2019]

addressed in the tutorial_solutions branch:

• Include solutions to a new branch... (let me know if you'd prefer multiple branches, though I like the idea of one solutions branch)
• Since the data entries in histogram are pretty boring, maybe enhance them with some measure of central tendency and some measure of spread (I ended up tracking the most recent 5 data points, though this can be changed to the most recent x data points if 5 is too few)

[Crista2019]

tasks left TODO:

• ask @brohitbrose about what behavior should go into the logic of histogram.didRemove((k,o) ... to update the stats lane(s) in swim\tutorial\UnitAgent.java (on the tutorial_solutions branch)

• make example changes to the UI code to reflect the suggested server-side changes (on the tutorial_solutions branch)

• consider how to update the way in which data is randomly generated in swim\tutorial\DataSource --that is random yet predictable

[Crista2019]

changes on the tutorial_solutions branch I have committed today:

• all data going to the stats_* lanes have been consolidated and sent from histogram to one stats ValueLane<Value>
• all UIs have a dropdown menu to select which web agent is shown
• changes between web agents are reflected visually with different color schemes
• the data sent from DataSource to each web agent are distinct so switching between them is more interesting
• the gauge UI now links to the stats laneURI and shows mean and local mean, variance, std dev