Inclusion of <unistd.h> in src/nepy/nepy.cpp prevents installing calorine on Windows

Hi developers,

I'm trying to install calorine on Windows, but it fails because of the #include <unistd.h> in src/nepy/nepy.cpp:

<unistd.h> is a POSIX-specific header that does not exist on Windows. I tried to remove it and install caroline from source, and it worked well on my windows.

If it is not necessary, would you consider removing it? This small change would make calorine much more accessible to Windows users.

Thanks a lot for your work on this package – it's very useful for materials modeling!

Best,

Zihan YAN