This pull request adds utility functions to set and get pixel colors on an M5Stack Atom Matrix device by x, y position.

Valid values of x and y are 0-4 inclusive.

Tested on an M5Stack Atom Matrix as follows:

>>> import atom
>>> a = atom.Matrix()
>>> a.set_pixel_color_x_y(0, 0, 16, 0, 0)
>>> a.set_pixel_color_x_y(4, 4, 0, 16, 0)
>>> a.get_pixel_color_x_y(0, 0)
(16, 0, 0)
>>> a.set_pixel_color_x_y(5, 0, 16, 0, 0)
>>> a.set_pixel_color_x_y(0, 5, 16, 0, 0)
>>> a.get_pixel_color_x_y(6, 6)
>>> a.get_pixel_color_x_y(-1, -1)

Follows convention established in atom.py that invalid values for x and y do nothing.

Image showing the code above running on a device...