Create an example for joint inversion using linear physical relation constraint

[xiaolongw1223]

Summary

Hey, it's been a while since I submitted a PR :). Just create a quick example for the joint inversion using linear physical relation constraint and potential-field data. All inputs have been prepared, could someone please assist with uploading to the storage place? Thanks~ Let me know if any changes are required.

PR Checklist

• If this is a work in progress PR, set as a Draft PR
• Linted my code according to the style guides.
• Added tests to verify changes to the code.
• Added necessary documentation to any new functions/classes following the
  expect style.
• Marked as ready for review (if this is was a draft PR), and converted
  to a Pull Request
• Tagged @simpeg/simpeg-developers when ready for review.

Reference issue

What does this implement/fix?

Additional information

[santisoler]

Hi @xiaolongw1223! Thanks a lot for opening this PR and sorry for the delayed reply.

I'll take a look a the example and leave a review.
I think @dccowan would be interested in taking a look at this as well.