Add Setup and Start shell scripts for a virtual Python environment on Linux, add EOX with 2021 images as provider

[JT8D-17]

This adds:

• A bash script to create a virtual Python environment in the O4XP folder. This is intended for some Linux distros like Arch Linux that are already using Python 3.12 by default and to work around current scikit-fmm versioning issues. The script will install all required PIP packages and download, compile and install scikit-fmm from a branch that works in 3.12.
• A bash script that will start O4XP in said virtual Linux environment.
• A modified requirements.txt that is not version locked.
• A new provider file for the superior 2021 imagery from EOX (as opposed to the default 2018 ones supplied by EOX2), see https://s2maps.eu/

[w8sl]

Self contained O4XP install is a genial idea, especially when running and testing several versions of O4XP - thank you!

System GDAL is not visible in venv. pip install gdal==$(gdal-config --version) works within Python 3.12 venv on macOS and GDAL 3.9 gdal_array, used by O4XP, works correctly only in venv created with "--system-site-packages" option with GDAL 3.8.5

$PWD is a bit different on Arch SCRIPT_DIR=$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" &> /dev/null && pwd) works on all Unix based systems. path=PATH on macOS, it is better to use different name, like "venv_path"

idna, charset-normalizer, urllib, certifi are installed automatically by pip, as dependency of requests. Chardet is deprecated, if I understand correctly (replaced by charset-normalizer)

[w8sl]

scikit-fmm for python 3.12 has been released. Use: pip install scikit-fmm. On macOS, Arch, Ubuntu 24 it is only possible in venv (tested), as global pip/pip3 install has been disabled with Python 12.

[w8sl]

If I understand correctly, it is still compiled from source - by pip install.

[JT8D-17]

Scikit-fmm installs from pip without issues for me now. I've updated the script and requirements accordingly.