text: Title: [ENH] Count the cumulative number of unique elements in a column
  Body: # Brief Description
  
  I would like to propose a function that counts the cumulative number of unique items in a column.
  
  # Example implementation
  
  ```python
  def cumulative_unique(df, column_name, new_column_name):
      """
      Compute the cumulative number of unique items in a column that have been seen.
      
      This function can be used to limit the number of source plates
      that we use to seed colonies.
      """
      df = df.copy()
      unique_elements = set()
      cumulative_unique = []
      for val in df[column_name].values:
          unique_elements.add(val)
          cumulative_unique.append(len(unique_elements))
      df[new_column_name] = cumulative_unique
      return df
  ```
  
  I'm happy for anybody else to run with the implementation above and improve upon it.