Measures of Center Rewrite

[retabak]

• misconceptions json
• objectives json
• Rewrite will separate review / remediation from instruction on choosing the best measure of center.
• Interpreting section will include instruction on choosing the best measure of center and connecting measures of center with histograms.
• consider teaching measures of center before histograms #2197
• rewrite the mode(s) section of measures of center to teach about mode(s) of data that isn't numerical #1839
• It would be useful to have a visual representation of the median
  -- a depiction of students lined up by height with a line dividing them into halves
  -- skyline of tall buildings shortest to tallest

[retabak]

accidentally tagged the commit that closes this issue to #2229