Restructures the code into a package so that it can be imported by other software. I did not alter the code beyond moving files around and fixing the imports.

• Restructures the repository into a python package called building_metrics_3d
• Moves the contents of requirements.txt to pyproject.toml
• Adds tox for testing the installation
• Fixes the tests for using the new package

I tested with python3.11

Example module import of the package from building_metrics_3d import shape_index

Fixes #16