Option of defining an environment script for starting engines (with SSHEngineSetLauncher)

先阅读 SSHEngineSetLauncher 和 SSHLauncher，然后检查 issue 中描述的小型跨平台测试脚本以及 subprocess.Popen 的环境处理。完成的标准是：环境脚本能够在 Linux 和 Windows 上准备 engine 环境，同时不让 Python 进程句柄指向 shell，并包含实现测试和文档。

We need to adapt the environment before starting the actual ipp engine (through the SSHEngineSetLauncher). Adapting the environment via the environment parameter (in class SSHLauncher) is possible, but it's not very practical, since the launcher needs to know all the engine computer details in advance.
Calling a local environment script before starting the engine would give much more flexibility and would easily allow starting the engine e.g. in a python virtual environment, conda environment, etc.

Although the general idea is quite simple the actual implement is not as easy. The problem: one cannot simply start a shell, next the environment script and then the actual command because the process handle (in python) would point to the shell and not the actual command. Hence, the retrieved process id would be incorrect. Using some system code it would be still possible to retrieve the true process id, but this is especially difficult in windows which is why I tested an alternative solution:
Starting a shell, outputting all environment variable, running the environment script and outputting all environment variable again. A bit of python code analyses the output and collects the environment variable changes (especially added and/or changed env.vars). I have implemented a small test script that works under linux and windows.
The collected changes can then be easily passed to the env parameter of subprocess.Popen function.

Does this sound like an appropriate idea and solution?
As before I'm happy to provide the implementation, tests, documentation, etc.