[WIP] update continental extension cookbook

[naliboff]

This is a work in progress PR to update the continental extension cookbook.

The changes include:

1. Multiple updates to the model design, include the use of active particles, improving the readability of the material model, and updated flow law parameters.
2. Addition of a script to illustrate how the flow law prefactor scaling values are calculated

The above changes do affect the model dynamics, with an example image of the viscosity after 5 Myr included below.

@gassmoeller (and others) - I thought it would be easiest to discuss changes to the PRM file first before updating the documentation files.

Before your first pull request:

• I have read the guidelines in our CONTRIBUTING.md document.

For all pull requests:

• I have followed the instructions for indenting my code.

For new features/models or changes of existing features:

• I have tested my new feature locally to ensure it is correct.
• I have added a changelog entry in the doc/modules/changes directory that will inform other users of my change.
  

[gassmoeller]

I think the changes make sense. Go ahead with the rest of the update.

[gassmoeller]

@naliboff can you remind me if this rework is necessary for the release? I just reran the current continental extension cookbook and it at least converges pretty fine and looks pretty similar to what we have on the website:

My result (slightly different colorscale):

Fig. 105 in Documentation

The reason I am asking is that this is a pretty substantial rewrite of the cookbook, and I would like to create the release soon. Does it matter if this rewrite goes into the release or is merged afterwards?

[naliboff]

@gassmoeller - Thanks for checking the model on your end. My motivation for including this PR in the release was to highlight the updated solver settings, use of particles, and formatting of the material model section. Incremental, but certainly very useful improvements. Absolutely ok if this is merged after the release, with the only caveat being that this is the version of the cookbook we will want to use in future tutorials (i.e., will it be a problem to use the main branch for the hubzero notebooks)?

[gassmoeller]

I guess either way is fine with me, I would just like to get the release out in the next 1-2 weeks, and all other features we wanted are already merged so we can test them now. Changing something right before a release has the risk of requiring a point release to fix unrecognized bugs. I would suggest we just proceed with this PR and see when it is ready, if before the release I am ok including it, if afterwards I see no point in delaying the release further.