Adds constructors for all classes in the hierarchy

[hlapp]

This follows the initial example of using the New() convenience function and the nexml.tree() constructor function that wraps that, and expands it to all other classes in the nexml class hierarchy. These constructors are now also exported so that users of the package can program with them while being insulated from the exact inner workings of initializing and naming classes.

The only classes not receiving a constructor function are the ListOfXXX classes, which are simply glorified lists but not otherwise embedded in the hierarchy (they also don't inherit from Base).

Also adds documentation, specifically here for the toplevel class (nexml). (It turns out that once you start documenting R CMD check pulls you into a bottomless documentation pit, because now you can't even miss a single slot, accidentally or on purpose (internal use slots, anyone? 😞)

[hlapp]

Notes:

• I investigated where patch coverage is falling short. Almost all areas of the patch that aren't getting hit by tests were already not getting hit by tests, so this isn't really a change in coverage. (This isn't arguing that it's fine that those areas aren't getting hit, but I think fixing test coverage deserves its own issue and pull request.)
• I have not taken out the rootEdge class (see #207) here. I could include that here, though its effect on patch coverage is minor (only a single line of that block is being changed here).

[cboettig]

This looks good to merge! Yeah, we can drop rootEdge here or in a separate PR if you prefer.