load_PGM: takes a filename (string of a PGM image file as input, loads in the file, and returns the image matrix specified by the file. If the file is not in valid PGM format, then raise an AssertionError with an appropriate error message. Note: For this and further examples, we put each sublist on its own line for readability purposes. When you write your examples in the docstring, however, the output must be all on one line for the doctest to work properly. >>load PGM(comp.pgm [[0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0], [0,51,51,51,51,51,0,119,119,119,119,119,0,187,187,187,187,187,0,255,255,255,255,0], [0,51,0,0,0,0,0,119,0,0,0,119,0,187,0,187,0,187,0,255,0,255,0], [0,51,0,0,0,0,0,119,0,0,0,119,0,187,0,187,0,187,0,255,255,255,255,0], [0,51,0,0,0,0,0,119,0,0,0,119,0,187,0,187,0,187,0,255,0,0,0,0], [0,51,51,51,51,51,0,119,119,119,119,119,0,187,0,187,0,187,0,255,0,0,0,0], [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]])