Allow for a simple way to Set() without traversing descendants

[telenskyt]

Please allow for a simple way to Set() attributes of a single node without traversing its descendants (e.g. by adding an option traversal="none" to the Set() method).

You might argue that it could be set directly like in #125 :

data(acme)
acme$IT$name <- "ITNEW"
acme$IT

however, the clean OOP design is to use methods Get() and Set() instead of directly modifying the objects. User should then never directly assign to attributes, but do it this way instead:

acme$IT$Set(name = "ITNEW", traversal = "none")

This is a clean OOP approach, and it would also allow to solve the #125 very elegantly by resolving this in the Set() method!

[telenskyt]

Perhaps even better solution would be to add new method, e.g. SetNode(), that would do the same thing without traversing and without additional option.