set_pixel

[untzag]

it's time to start actually introducing code to address our LEDs

in the long run, we want to use simple setitem and getitem syntax (see #9)

as a stepping stone, I propose we create a very simple set_pixel method with arguments (i, j, color)

i, j integers, color hex string

this will allow us to focus on the simple problem of addressing the LEDs as separate from the syntactic sugar that we want to introduce