[GRASS translations] Greek po files added

[Carlos Dávila]

Hello all, and Greek translators specially
I have just sent Markus all grass*_el.po files, so they will be soon in 
cvs. It's necessary to check encoding, as neither Markus nor me were 
sure about the right one for Greek. I set it to ISO-8859-7. If necessary 
it can be changed editing files headers by hand with any text editor.

Regards,
Carlos