add a helper from_dict to easier create upgrade instances with a model link
promote scent trails to an object attribute, for all objects
carry the player through level warps
fix changed cost.py import
avoid zero division displaying stats
accommodate sprite positions relative to our play_image vs the viewport, i.e. a single screen image mapped to a larger virtual space.
add a debug command to become any other random character
add test-map.tmx for demoing map building features
remove testing stuff