'Two required Coordinate objects defining tile pyramid bounds.
Boundaries are inclusive: upper_left_high is the left-most column,
upper-most row, and highest zoom level; lower_right_low is the
right-most column, furthest-dwn row, and lowest zoom level.'

def __init__(self, upper_left_high, lower_right_low):

    self.upper_left_high = upper_left_high
    self.lower_right_low = lower_right_low
