class_docstr: Count occurences of a value or values.
  
  This aggregation can be used in three ways:
  
  1. If no arguments are specified, then it will count the number of rows in the table.
  2. If only :code:`column_name` is specified, then it will count the number of non-null values in that column.
  3. If both :code:`column_name` and :code:`value` are specified, then it will count occurrences of a specific value.
  
  :param column_name:
      The column containing the values to be counted.
  :param value:
      Any value to be counted, including :code:`None`.
code: def run(self, table):
      if self._column_name is not None:
          if self._value is not default:
              return (
                  table.columns[self._column_name]
                  .values()
                  .count(self._value)
              )
          return len(
              table.columns[self._column_name].values_without_nulls()
          )
      return len(table.rows)
  
file_path: agate/agate/aggregations/count.py
imports: from agate.aggregations.base import Aggregation
  from agate.data_types import Number
  from agate.utils import default
label_desc: Use image node_id 3 for calling the Count obj's underlying member method code with example usage: obj.run(table) and returns: len, len, table