improve Annotating program

[mixmixmix]

• turn into a py program so it can be easier for version control
• allow going to previous frame
• allow for removing all existing boxes (as sometimes they are not helpful)
• change display of boxes into multicolour, possibly a bit thinner line

[mixmixmix]

#18 - now it is in file annotate.py and allows for running without showing pre-trained boxes (which I found helpful with my often-overlapping fish data)