Simple import handling for when flowslicer is installed as a package

[jprokos26]

Allows python -c “import flowslicer” to be run from either the local directory without any package installation (existing functionality) as well as supporting flowslicer to be installed as a package to be run anywhere (new functionality).

This PR does not include the setup.py file necessary for generating the package as I installed this as a submodule within a separate package using setuptools.find_packages(). See this example to see how I included flowslicer within my own setup.py.
Note, a better/clearer change would be to set up flowslicer to be a python package such that the try block is unnecessary but I did not want this PR to change any existing functionality, only to add new.

[jprokos26]

To clarify why I needed this change, I could not find an easy way to call flowslicer within BinaryNinja to view the output of calling DataFlowILFunction.graph(). Getting it running within binaryninja was my solution for which I have not determined if the graph part works.