Hotfix/24 empty assertion error when using np.array

[boludo00]

This closes #24 by adding a different check

for node in nodes:
            # check if node is `number-like`
            try:
                node = int(node)
                self.add_node(node, **nd[node])
            except:
                # or node could be string
                assert isinstance(node, str)
                self.add_node(node, **nd[node])