add icesat2py clarification note

[JessicaS11]

Per this great review by @dimejimudele, I've updated the origin and purpose and readme to clarify the distinction between icesat2py and icepyx. Happy to be told I've missed the mark and take suggestions for improvement.

[github-actions[bot]]

:point_left: Launch a binder notebook on this branch for commit c62bf0ac114e7b0774161a6825b6a032d37e638e

I will automatically update this comment whenever this PR is modified

:point_left: Launch a binder notebook on this branch for commit ebb00aa60ba164adcbf32600a2ce946ae75bb61f

:point_left: Launch a binder notebook on this branch for commit e5a946cf555fbab4f743fff10eefd7cec160d44f

:point_left: Launch a binder notebook on this branch for commit 8a725d0b5c2cf1fbe6b2492f8a077d0f26cd7e08

:point_left: Launch a binder notebook on this branch for commit 47c41194598c79348f201cd7268d839f543b0b6b

:point_left: Launch a binder notebook on this branch for commit 26680bfdf4b4688adb660dd7671cce0c0df09d26

[codecov-commenter]

Codecov Report

Base: 58.29% // Head: 58.29% // No change to project coverage :thumbsup:

Coverage data is based on head (e5a946c) compared to base (01facea). Patch has no changes to coverable lines.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #395 +/- ## ============================================ Coverage 58.29% 58.29% ============================================ Files 31 31 Lines 2247 2247 Branches 433 433 ============================================ Hits 1310 1310 Misses 854 854 Partials 83 83 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=icesat2py). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=icesat2py)

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