class_docstr: Calculate the mean of a column.
  
  :param column_name:
      The name of a column containing :class:`.Number` data.
code: def __init__(self, column_name):
      self._column_name = column_name
      self._sum = Sum(column_name)
  
file_path: agate/agate/aggregations/mean.py
imports: from agate.aggregations.base import Aggregation
  from agate.aggregations.has_nulls import HasNulls
  from agate.aggregations.sum import Sum
  from agate.data_types import Number, TimeDelta
  from agate.exceptions import DataTypeError
  from agate.warns import warn_null_calculation
label_desc: Use image node_id 1 to create a new Mean object from inherited base classes: Aggregation with example: obj = Mean(column_name)