def range_for column_name, field_name
      column = @@bitfields[column_name]
      raise ArgumentError, "Unknown column for bitfield: #{column_name}" if column.nil?
      return column[field_name] if column[field_name]
      raise ArugmentError, "Unknown field: #{field_name} for column #{column_name}"
    end
def range_for column_name, field_name
      column = @@bitfields[column_name]
      raise ArgumentError, "Unknown column for bitfield: #{column_name}" if column.nil?
      return column[field_name] if column[field_name]
      raise ArugmentError, "Unknown field: #{field_name} for column #{column_name}"
    end
Returns the column by name

 +column_for

 @param [ Symbol ] column_name column name that stores the bitfield integer