Revise 04-VelocityBasics.html in accordance with feedbacks from M.S.

[michaelchin]

simplify this by only showing the following plots:

• Calculating average global plate velocity through time
• Visualising PlateReconstruction velocity data
• Plotting a velocity streamplot (OK but would be good to add a sentence description of what a stream plot is)
• Create a velocity animation (but the animation in this example doesn’t work)
• Calculating velocities of reconstructed points using the Points object

After the Visualising Points object velocity data section things start getting confusing

Velocities at a past time - shows a non-working example whereas the next one titled Dealing with oceanic points, this shows a working example. Can we please merge these together so that we only show the working example. It should just be titled Velocities at a past time

Plotting past velocities at present day - is confusing and I wonder if we even need to show the example. Maybe Dietmar knows if this is useful? In fact anything after this might not be useful? Dietmar?

[jcannon-gplates]

@michaelchin, can you forward me the email Maria sent? I'm the one responsible for the section she's referring to.

[michaelchin]

@jcannon-gplates I can fix the animation not showing problem. Do you want me to fix that?

[jcannon-gplates]

Sure, thanks!

[jcannon-gplates]

Thanks for the fix in 72b02b5. The only slight issue would be the frame-per-second is faster when rendered to html (judging from the html version you emailed me). But it's no big deal, we can leave it as-is.

And, as discussed with Maria, I'll separate this notebook into a basic and advanced version (to avoid removing documentation that's useful in advanced scenarios).