Add documentation and examples to all the utility methods

Right now a variety of our pyttb_utils doc strings are lacking sufficient information. For example [ind2sub](https://github.com/sandialabs/pyttb/blob/70fc7c7f3d697950ed5b27fcb7f91cb5df3c6f38/pyttb/pyttb_utils.py#L504-L516)