Fix for issue 104

[afisher1]

fixes issue #104. To test you must not have python helics package installed in your system level python. Create a python virtual environment with the helics python package. Activate your virtual environment and run a python helics federate using the helics runner. if it runs successfully it used the active virtual environment.

[nightlark]

This hard-coded changing of "python" to sys.executable is problematic when using a helics runner/cli installed by pipx, and any other scenario where we want to run a "python" binary other than the one the helics runner itself is installed in -- which I can see causing other bug reports down the line that would result in needing to revert the proposed fix.

From reading python/cpython#86207, it sounds like this issue comes down to Windows having search path rules for binaries that are different than POSIX, behaving in an unintuitive way with the Python virtual environment redirection script.

Using https://docs.python.org/3/library/shutil.html#shutil.which to make the search behavior for the executable to run consistent across platforms seems a bit better conceptually, though still doesn't fully resolve the issue that at least for now when a user is using Python it is best to just fully specify what virtual environment is being used to make it clear what is happening (TODO: update documentation).

[afisher1]

I agree that shutil.which is the better route thanks @nightlark. Can you take another look? I was unsure of how we implement error handling in cli.py. I'm unfamiliar with click. Not sure we should be handling raising errors through click or not.