Registering a root node if one already exists

[codan84]

To replicate:

const { Lrud } = require('lrud')

const navigation = new Lrud()

navigation.registerNode('root', { orientation: 'vertical' })
navigation.registerNode('root', { orientation: 'horizontal' })

console.log(navigation.tree)

Problem: trying to register a root node again (with the same id, orientation etc doesn't matter) does not change the existing root node to the new one. Instead, it adds another node at the root of the navigation tree, except the node is added with an undefined id.

Expected behaviour: either throw an error to say this node already exists (make it explicit that we can't do it), or unregister existing root node (see other issue I submitted!) and register new one in it's place.

[thomascgray]

@codan84 thanks again for finding & raising! After a bit of an internal discussion, we also agree LRUD should throw an exception in this instance. Getting a fix out now.