Statistics features for Enumerable

 Enumerable::Statistics provides some methods to calculate statistical summary
 in arrays and enumerables.
 .
 The following methods are supplied by this library:
  - `Array#mean`, `Enumerable#mean` - Calculates a mean of values in an array
    or an enumerable
  - `Array#variance`, `Enumerable#variance` - Calculates a variance of values
    in an array or an enumerable
  - `Array#stdev`, `Enumerable#stdev` - Calculates a standard deviation of
    values in an array or an enumerable
  - `Array#mean_variance`, `Enumerable#mean_variance` - Calculates a mean and a
    variance simultaneously
  - `Array#mean_stdev`, `Enumerable#mean_stdev` - Calculates a mean and a
    standard deviation simultaneously
  - `Array#median` - Calculates a median of values in an array
  - `Array#percentile(q)` - Calculates a percentile or percentiles of values
    in an array
  - `Array#value_counts`, `Enumerable#value_counts`, and `Hash#value_counts` -
    Count how many items for each value in the container
  - `Array#histogram` - Calculate histogram of the values in the array