improve sample/prep info summary

[antgonza]

From qiita.admin email discussion:

• Multiple columns that have similar groups/counts: avoid duplication of columns
• Columns that have multiple groups and a few groups have much lower
  counts than others: this should help find misspellings or odd groups.
• Columns that all values are the same.
• Columns that all values are different except 10% of the rows