Add cache mechanism to graph nodes

[nvaytet]

Fixes #27

Small benchmark:

import plopp as pp
from plopp.widgets import SliceWidget, slice_dims, Box
from plopp import figure, input_node, widget_node, node, Node, View
import scipp as sc

N = 10000
M = 10000
xx = sc.arange('x', float(N))
yy = sc.arange('y', float(M))
da = sc.DataArray(data=xx, coords={'x': xx})
db = sc.DataArray(data=yy, coords={'y': yy})


class DataView(View):

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.data = None

    def notify_view(self, message):
        node_id = message["node_id"]
        self.data = self.graph_nodes[node_id].request_data()


a = input_node(da)
b = input_node(db)

c = node(lambda x, y: x * y)(x=a, y=b)

nodes = []
views = []
for i in range(20):
    nodes.append(node(lambda x: x[x.dims[-1], 0])(x=c))

for j in range(2):
    for n in list(nodes):
        for i in range(2):
            nd = node(lambda x: x * (i + j))(x=n)
            nodes.append(nd)
            views.append(DataView(nd))

print(views[-1].data)

a.notify_children(message='hello from a')

print(views[-1].data)

Before:
real 0m17.526s
user 0m28.001s

After:
real 0m1.117s
user 0m1.638s

I saw no difference in RAM usage, but maybe we can think of an example where more RAM would be used?
In all the ones I tried, references to large data were being kept by either the views/figures, or the children nodes themselves.

[jl-wynen]

I think you could do away with _needs_update and use _data is None to indicate that the data is out of date. This would prevent accidental use of outdated data.

[nvaytet]

Nice suggestion thanks.