use assignment by reference instead of pointers

[robUx4]

No need to dereference a pointer.

[mbunkus]

While I agree that you can just cast the reference, you don't need to cast for the vast majority of these cases in the first place. Just use elt.SetValue(…) for all those places where the only use is to assign values. The casts are only there from way back when the SetValue() & GetValue() functions didn't exist.

[robUx4]

Ah, I thought the operator was the preferred way. I had some troubles with it and ending up removing the factorization of those for now. If you think it's better not to have them at all, I can remove them for good.

[mbunkus]

We can leave the existing operator <whatever>() in the classes for compatibility purposes, but we should not really use them inside the library anymore. I'd even be fine with removing them altogether, even the member functions themselves.

[robUx4]

Closing in favor of #136