Consider the following example where a graph contains nodes but no views.
The leaf node c is just logging results to a container.

import plopp as pp
import numpy as np

results = []

rng = np.random.default_rng()

a = pp.Node(rng.random)
b = pp.Node(lambda x: -x, x=a)
c = pp.Node(lambda x: results.append(x), x=b)

a.notify_children("")
results

Without the changes in this PR, the node a would notify b which would notify c, but c is not a view and would not request data and its function (and the ones in b and a) would never be called.

Now, if a leaf node is notified, it automatically requests data from parents and makes a call to it's internal function.