More nhx stuff

[caseywdunn]

A few things:

• Added a drop.tip method

• @nhx_tags$node now numeric

• @nhx_tags sorted on @nhx_tags$node

Note that I added a generic for drop.tip. Noticed that you don't have generics on other methods, so not sure how you want to handle that.

Be well.

[GuangchuangYu]

I detect several issues:

1. broken existing code since no drop.tip method for phylo.
2. a bug as you forget to update the node number in nhx_tags
3. over-write node.label if exists
4. forget to remove node.label if previously NULL.

All were fixed in 0b9c272.

[caseywdunn]

Great catches - thanks for sorting out those issues. Looks good now.