Changed 'add_base_attribute' method name into 'set_base_attribute'

[kritchie]

This PR addresses GH-117 by refactoring the method name add_base_attribute to set_base_attribute

Prior to this change, the add_base_attribute had an idempotent definition :

self.base_attributes = kwargs

While the method declaration let's the user think that this method is indeed not idempotent by having the keyword add in the method declaration. Setting it to set_base_attribute will remove potential errors from users.