12.6 Divergence Update

[nelong]

• [ ] Make plots in Preview Activity easier to read labels of points
• [ ] Move 12.6.1 to technical section at end and add brief description of meaning
• [ ] Look at how to split equations into multiple lines
• [ ] Add short example with a calculation of divergence and show how that measures a change in strength of the vf at couple different points
• [ ] Use thicker lines in colored pictures (like in curl section)
• [ ] Add subsection at end with content dependencies and notes to instructor
• [ ]