Add project-specific object detection module.

The new module is called obj_baxter, which contains methods that would
be useful specifically to our CS473 project. By only requiring image
paths, it allows the user to take advantage of obj_detect and its OpenCV
methods without getting lost in the details.
Make getter methods safer.

As the user is not required to load images of the box, uncompressed
object, or compressed object immediately, the getter methods for the
dimensions of these things are set to return a size of (0,0) by default.