Improve environment setup & fix path errors & exec

[malcolmgreaves]

Added instructions on creating a virtual environment with conda.
Obviates the need to use python3/pip3. Also ensures that the
requirements installation will work: there won't be any conflicts
in the environment.

Fixed the path import errors by adding src/__init__.py, explicitly
importing: from src import .., and by moving the executable Python
scripts to the project's root. Updated README documentation too.

Made the download_model.sh script exectuable and changed it to
use the first bash executable found on the system's $PATH.
Additionally, made the Python scripts not executable (they
should be run with python ... at any rate).

[malcolmgreaves]

This PR also fixes #7, which overlaps a bit with #8. However, this PR takes a different approach, instead making src a python package and having the executable files live outside the package.

[nkconnor]

probably more sustainable to add a scripts directory. also src would typically be named something like gpt

[nealmcb]

I find pip a much less intrusive requirement than conda, though I know conda is popular in many circles.
I'd suggest instead sticking with a standard pip aproach, and recommending in the README that people follow common best practices by setting up a venv or conda virtual environment before doing the installs.

[WuTheFWasThat]

yeah I'll just recommend using a virtual environment in the README (personally I like pipenv!)