Add SlowFast as an optional dependency

[zhong-al]

#76 removes SlowFast as a dependency since SlowFast cannot be installed on Windows computers because it requires detectron2. However, it makes sense to keep SlowFast as an optional dependency to have access to a greater variety of model architectures.

Update install to be
pip install git+https://github.com/Imageomics/kabr-tools
or something like
pip install pip install git+https://github.com/Imageomics/kabr-tools.git#egg=kabr_tools[slowfast]