Add Sen2Cor Windows support
Thanks to the use of portable libraries, most of the sen2chain features should work on Windows. At the moment, the main brake is the way in which the Sen2Cor processor is called for the processing of L1C products.
On GNU/Linux, the L2a_Bashrc is sourced so that the Sen2Cor environment variables are set and the L2a_Process command is available. The function is then called with subprocess.
On Windows, it's the L2A_Process.bat file that is used. It should be used in a similar way.