Restructured code and other small changes

[ChiCheng45]

Description of work
Restructured the code so that it follows the src-layout as shown in https://setuptools.pypa.io/en/latest/userguide/package_discovery.html which is more standard than the previous layout. Previously I was having some issues getting the editable build to work, the restructuring fixes this. Also included some changes which improve things a bit which includes:

• updated gitignore to ignore: distribution/packaging files and directories, generated pyd extensions, pycharm related stuff
• moved package data to the manifest.in
• changed script to entry-points so that they can be run as if they were executables
• updated the github wheels.yml to use python -m build since python setup.py bdist_wheel is deprecated https://packaging.python.org/en/latest/discussions/setup-py-deprecated/

Fixes
Fixes MDANSE and MDANSE_GUI installation using editable mode

To test
Run pip install -e . in the MDANSE and MDANSE_GUI and checked mdanse.exe and mdanse_gui.exe run without crashing. Built wheels with python -m build and installed them in a different environment to check building is done correctly and checked mdanse.exe and mdanse_gui.exe again.

[MBartkowiakSTFC]

Excellent work! Works on my computer, and should be a big improvement on Windows machines.