Basics of Inversions

[thibaut.astic]

Pitch idea: bringing the Basics of Inversion section we did for GIFtools into the SimPEG documentation website or tutorials…

For reminder: https://giftoolscookbook.readthedocs.io/en/latest/content/fundamentals/index.html

Any thoughts? @lheagy @sgkang09 @doug

[lheagy]

This would be great @thibaut.astic! I think it would be especially impactful if we can interweave some code examples throughout