Add atom object name on the `cmd.get_model()` returned model

[pslacerda]

Now cmd.get_model() returns an atom model with the object name on it. I didn't want to use the object variable because it is a built-in, however I thought that any other name would be misleading.

[JarrettSJohnson]

Aside from the compilation errors, I'm wondering if this is the right approach. Wouldn't each atom's object name be duplicated (wasted space)? Would it make sense to have the name be stored per-object/model rather than per-atom?

[pslacerda]

I had this doubt and thought that resn is duplicate all over atoms, specially for polymers and decided to replicate.

And I'm not sure if the same happens when the string is built from the C API, but Python's strings of the same value share the same memory space.

[pslacerda]

I decided to return the object name on the atom model because it's convenient. It would be even more if it worked on iterate.

[JarrettSJohnson]

Thanks for the PR!