implemented tin (NIF) generator for pt_PT provider

I'm not really sure about this but I don't see a better way to get this right now.
What do you think?

Meanwhile, I'll try to get some kind of formula to generate this but I'm not that good at math and I couldn't find anything like it at google neither.

I did not add tests because it is already generated value is already tested. But I welcome suggestions to improve this.

Thanks in advance!