Boost::filesystem in PCL

Context

Currently, PCL uses the filesystem library from Boost. In the long term, we would like to replace all functions and classes from Boost::filesystem with something else and remove this dependency.
To find out where PCL uses Boost::filesystem, you can e.g. use grep -ri "boost.*filesystem" *
In C++17, there is a std::filesystem library which could replace the one from Boost, but at the moment, PCL still aims to be compatible with C++14.
In Boost 1.83.0, some functions from Boost::filesystem are deprecated. PCL should not use these deprecated functions any more. (see here, choose any build, select a macOS or Windows job, click Build Library, and search for filesystem)

Idea 1

Check if, instead of using functions and classes from Boost::filesystem, the same goal can be achieved in a different, C++14 compatible way (meaning, without using std::filesystem). This might for example be possible when simply checking the filename extension. In other situations, it may not be possible to find a reliable alternative on all platforms.

Idea 2

For the functions deprecated in Boost 1.83.0, that are used in PCL: check the alternative suggested by Boost. In which older Boost versions is this alternative available? PCL currently requires at least Boost 1.65.0.

Idea 3

If a function or class from Boost can not easily be replaced without using std::filesystem from C++17 (idea 1), then use std::filesystem like this:

#if (__cplusplus >= 201703L)
// TODO: new implementation using std::filesystem
#else
// existing implementation using boost::filesystem
#endif

This way, PCL stays C++14 compatible and we can automatically switch in the future.

If someone wants to work on this, please make sure that your pull requests are not too large (otherwise split them), and comment here first what you are planning to do.

Start by running grep -ri "boost.*filesystem" * and catalog the uses, then check the Boost 1.83 deprecation warnings and suggested alternatives. Confirm which replacements support the minimum Boost 1.65 and C++14; where necessary, preserve the Boost path for older standards and use std::filesystem only for C++17. Comment with a focused plan first and split the work into small pull requests.