Add atan2(nn) and deprecate atan(nn)

[vurvdev]

Too much confusion has been caused by whoever decided to make atan2 just an overload of atan in the discord (and probably to a bunch of people who haven't brought it up there either.

I propose moving the behavior to an atan2(nn) overload and deprecating the current function.

Could also make it return radians instead of degrees.