consistency with s3 generics

[lukejharmon]

Travis throws an error about the treedata s3 functions:

• checking S3 generic/method consistency ... WARNING print: function(x, ...) print.treedata: function(tdObject, ...) reorder: function(x, ...) reorder.treedata: function(tdObject, order, index.only, ...) filter: function(.data, ...) filter.treedata: function(tdObject, ...) mutate: function(.data, ...) mutate.treedata: function(tdObject, ...) select: function(.data, ...) select.treedata: function(tdObject, ...) summarise: function(.data, ...) summarise.treedata: function(tdObject, ...)

See section ‘Generic functions and methods’ of the ‘Writing R Extensions’ manual.