Add Python pathlib support for file IO

[sitic]

Adds support for pathlib.Path/os.PathLike type arguments to IO functions. E.g:

from pathlib import Path
import open3d as o3d

folder = Path('.')
mesh = o3d.io.read_triangle_mesh(folder / 'test.ply')
o3d.io. write_triangle_mesh(folder / 'test.ply', mesh)

is now supported in addition to str-type arguments.

Type

• Bug fix (non-breaking change which fixes an issue): Fixes #
• New feature (non-breaking change which adds functionality). Resolves Add pathlib support to IO methods in Python #6344
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

Currently, only string-type filenames are supported in all IO functions. Python's pathlib is quite popular and should be supported. This commit adds support for all libraries which are compatible with os.PathLike as pathlib is.

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

Using std::filesystem::path as type for filename arguments allows pybind to support both pathlib.Path and string through <pybind11/stl/filesystem.h> (which I backported to C++14 with minimal changes). Unfortunately this means we have to call filename.string() in the wrappers when calling the open3d C++ code.

I've added support everywhere except for o3d.data classes and some o3d.t.io.*Sensor class functions.

The documentation shows will os.PathLike as type for filenames:

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[ssheorey]

Thanks very much @sitic . This will PR really makes using Open3D easier!
PS: Regarding C++17 - we are ready to switch to that. I think the only issue was MSVC errors when built in C++17 mode but they go away in c++-latest mode. Let's merge this PR with C++14 in mind, but OK to use C++17 in the future. We can help fixing any build issues.

[Kroppeb]

I love you. This is awesome!