Add .stl format support and exporting batch of meshes into a folder

[lyd405121]

Changes

1. Add two example files [example]
2. Add .stl input/output option [main.cpp]
3. Add option for exporting convex meshes with different file names into a folder [main.cpp]
4. [readme.md] update
5. [io.h/.cpp] add SaveStl/SaveStls function
6. [model_obj.h/.cpp] add LoadStl function, both ascii and binary are ok!

Why add these

• Easy for users to quickly break a concave mesh into different files with main.exe after compiling
• In robotics, .stl format can be found everywhere

Test

• I can't build python moudle in my pc, so sorry for not running the test!
• But I manaully test all the examples files, it works well
• I also test different input format and output format

Worries

• I use std::filesystem instead of boost::filesystem
• Because I found c++20 in complie options