README.md

* Remove references to ISIS3 and replace with references to ISIS

* Remove references to ISIS3 and replace with ISIS

* Updated "isis" and "Isis" to "ISIS"

* Update TestPreferences

* Update IsisPreferences

* Reverted changes from 'Isis to ISIS' commit to see if it fixes failing tests.

Co-authored-by: Stuart Sides <ssides@usgs.gov>