Get net id from the transformers

[Yona-TYT]

With this I can know if an electrical network is connected to a factory, simply comparing the id from the transformers until they are all the same, resulting in all the factories connected to the same network. 

[Dwachs]

I added some functionality to check this with r8960:

http://dwachs.github.io/simutrans-sqapi-doc/changelog.html

[Yona-TYT]

This is great.

I will solve the problem with my script, thank you very much!.