Tutorial content revisions

[elbeejay]

PR to close #62. This PR is intended to be merged after #63, and is the "content" side of the tutorial updates where #63 focused on ensuring they would run and the continuous integration jobs passed.

---

Notebooks to go through:

• [x] Step_1
• [x] Step_2
• [x] Step_3
• [x] EXAMPLE_Finding...
• [x] EXAMPLE_Fixing...
• [x] EXAMPLE_Gravity...