Notebook demonstrating ACS imaging exposure time calculations with consideration of CTE losses (#455)

* added acs_etc_cte notebook

* added requirements

* fixed mjd, reran ETC where needed

* minor typos fixed

* pep8 related fixes

* pep8 related fixes

* adding TOC and banners.

* test out acstools package change

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Co-authored-by: Alyssa Guzman <aguzman@stsci.edu>
Co-authored-by: gibsongreen <gibsongreen99@outlook.com>

Author: 3 people