(gnus-summary-sort-by-recipient): new function

[Uwe Brauer]

Hello 

I sometimes  find in handy  to sort my  mail (in the mail sent group),
by the recipient, which is  defined in the *to* field.

I tried to simply copy and modify the 
gnus-summary-sort-by-Autor function

But I failed.
Has anybody tried to write such a function.

Thanks

Uwe Brauer