Adding ability to change edgecolors and linewidths

Hi! I am currently working on some visualizations where I need to "grey out" some atoms to highlight others. This is currently possible by changing the alpha value of the rgba colors, however, there isn't currently a way to also affect the alpha value of the edgecolors. Now users can pass a list of edgecolors or a single edgecolor to use for the atoms. Just like the colors parameter, this is only respected when working with complete occupancy.

This change does not affect the default functionality, just allows for some additional flexibility. I have added a test to make sure that the edgecolors are used in the order received.

I have also added similar functionality to play with the linewidths, which again should not change the default function.

I should add that I use black to format my code, hence the extended argument format in the function calls. I have only retained this formatting whenever there are actual changes to the code.