Misc code improvements

[jctophefabre]

code

• In Filesystem and FilesystemPath classes, prefer using Filesystempath objects as arguments instead of string wherever relevant. Provide converting constructors in FilesystemPath for std::string and char*
• Adapt whole code to use the refactored string helpers (StringHelpers.hpp)

compilation warnings

• GDAL (see also Fix gdal warnings #1077)
• Qt : split behaviour, ... (ubuntu 22.04, macOS Ventura)
• SpatialUnit possible loop (macOS Ventura)
• boost (replace where possible?)
• 'visibility' attribute on non-class types (ubuntu 22.04)

CI

• Split single GitHub workflow into multiple workflow, one per architecture, then update README file to reflect this update