Use element_color node attribute in drawing

[schatzsc]

    We're currently not using the properties `element_name` and `element_color`. Is there a future use case for those?

Originally posted by @JanCBrammer in #81 (comment)

[schatzsc]

Some quick and dirty code how to pass on element_color to nx.kamada_kawai_layout(G):

node_color_list = []
for i in G:
    element_color = G.nodes.data('element_color')[i]
    if element_color == "lightgrey":
        element_color = "0xD3D3D3"
    color_code = '#'+element_color[2:]
    node_color_list.append(color_code)
node_size = 1 / G.order() * 10000
options = { "with_labels": False, "font_size": 18, "node_size": node_size, "edgecolors": "black", "linewidths": 2, "width": 2, "node_color": node_color_list }
positions = nx.kamada_kawai_layout(G)
pos_attrs = {}
for node, coords in positions.items():
    pos_attrs[node] = (coords[0], coords[1])
    node_attrs1 = nx.get_node_attributes(G, 'element_symbol')
    node_attrs2 = nx.get_node_attributes(G, 'atomic_number')
    custom_node_attrs1 = {}
    custom_node_attrs2 = {}
for node, attr in node_attrs1.items():
    custom_node_attrs1[node] = str(node)
    custom_node_attrs2[node] = attr
for node, attr in node_attrs2.items():
    custom_node_attrs2[node] = custom_node_attrs2[node]+"\n"+str(attr)
nx.draw(G, positions, **options)