Example noteboks are out of date

There are few places where examples in the notebooks are using wrong keyword parameters,
eg Part VI buffer

--------------------------------------------------
TypeError        Traceback (most recent call last)
<ipython-input-6-eb223277a679> in <module>()
      4 opens  = marble_stream('oo---------------------------------------------------|')
      5 closes = marble_stream('-------------------------c|')
----> 6 d = subs(xs.buffer(buffer_openings=opens, buffer_closing_mapper=lambda: closes))

TypeError: buffer() got an unexpected keyword argument 'buffer_closing_mapper'

group_by

--------------------------------------------------
TypeError        Traceback (most recent call last)
<ipython-input-8-cd035db1f337> in <module>()
     16 src = O.from_(codes).group_by(
     17     key_mapper     = lambda x: x[keyCode], # id of (potentially new) streams
---> 18     element_mapper = lambda x: x[keyCode]  # membership to which stream
     19 )
     20 # we have now 6 streams

TypeError: group_by() got an unexpected keyword argument 'key_mapper'

Find the Part VI buffer example and the group_by example in the notebooks, then read the current RxPY signatures for the parameters shown in each traceback. Run the affected notebook examples and update the outdated keyword arguments until both examples execute without the reported TypeErrors.