Add sensitizing rules

[jchester]

Add Western Electric and Nelson sensitizing rules. Note that for each, the first rule (no points outside 3σ) is already handled.

• Western Electric rules
  • Rule 2
  • Rule 3
  • Rule 4
• Nelson rules
  • Rule 2
  • Rule 3
  • Rule 4
  • Rule 5 (same as WE Rule 2)
  • Rule 6 (same as WE Rule 3)
  • Rule 7
  • Rule 8