Remove begin() and end() from DataFrame

[samsta]

It seemed like a clever move to make DataFrame behave like a container initially, but in hindsight I find it confusing as hell since the iterators only expose the data and not the ID. It also confuses gmock because it then thinks 'ah, I know how to print that, it's just a container', omitting - again - the ID, and ignoring whatever operator<<() we provide.

This is likely to be tedious that far into the project - but better late than never ;-)

[samsta]

This will have the welcome side effect (hopefully) that we don't need to do a str() when comparing frames in the unit tests.