add ~/ to "cd projects" command

Update donkey.md

Simplifying cd to projects dir

Remove deprecated message argument from pytest.raises().

i guess the "--tub" was missing for the "makemovie" command usage example

Fix greyscale image handling

Fix image handling (img_crop, normalize_and_crop, load_scaled_image_arr) to deal with greyscale images correctly.
Also various spelling fixes.