[REVIEW] Strange behaviours

[ClaudMor]

Dear authors,

While reviewing your package, I noticed the following unexpected behaviours:

1. Assigning strings to edges is allowed:

from pymnet import *
net = MultilayerNetwork(aspects=0)
net[1,2] = "hello"
net[1,2] 

"hello"

2. Modifying the MultilayerNetwork via a MultilayerNode:

from pymnet import *
net = MultilayerNetwork(aspects=0)
net.add_node(1)
net[1].mnet.add_node(2)
list(net)

[1, 2]

I understand that these behaviors do not break the software, therefore fixing them is not mandatory for this review. Anyway, for the future, I would like to respectively point out:

1. Disallowing this by e.g. throwing and error;
2. Using getters and setters, or name mangling;