Add the possibility to alias columns

It would nice to have the possibility to "alias" column names to impose some sort of lookup rules. This is fundamentally different from Define.
This would allow also to treat branch names which are not C++ names in jitted filters/actions.
The replacement needs to be done before the loop.
The method can be called "Alias"

// ...

tdf.Alias("myimpossiblename+which I chose for my__ branch:?", "my_branch_1");

The return value may will be the initial node itself.