12.4 Scalar Line Ints Update

[nelong]

• [x] Change 12.4.1 notation to be r_n rather than r_k
• [x] Change to rather than $ in caption
• [x] Add example for assessing whether line ints will be positive/neg/zero before Activity 12.4.2 (should have a visualization of a curve in space and an idea about whether the function is more positive or negative on this curve, maybe a couple examples. One where it is clear, one where the calculation is necessary because you can’t weigh length of path versus magnitude of function)
• [x] Split equation in Example 12.4.9 into multiple lines?
• [x] Add more detail to captions in subsection 12.4.3
• [x] Add opacity to surface in last plot
• [x] Add subsection at end with content dependencies and notes to instructor