Add ICESat2 ATL15 dhdt tutorial and supporting images (#107)

* waiting on PR review to finalize ATL15 outputs

* update tutorial with most recent version before Apr 5 PR merge to main

* remove extra note cell

* add new ATL15 dhdt tutorial notebook and supporting images

* clean up new atl15 notebook

* incorporate changes from tutorial co-author review

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Co-authored-by: jessicas11 <[email protected]>
Co-authored-by: Jessica Scheick <[email protected]>
Co-authored-by: Matthew Siegfried <[email protected]>