Added a setup.py file for a local raft package

[martin-chobanyan-sdc]

This pull request adds a setup.py file which allows the user to run pip install . from the root project directory. This locally installs a raft package. The user can then import the code as follows without needing to use sys.path.append:

from raft import RAFT
from raft.utils.flow_viz import flow_to_image

Note that the core directory has been renamed to be raft.