WIP: Allow private R and C members in nClass

[paciorek]

I think this is reasonably solid/complete, including testing.

Some comments:

• I did not set activeBindings for Cprivate fields. I think that makes sense, but it means that print(Cobj) in R won't show that a Cprivate field even exists.
• I do error-trap attempts to improperly access private members from outside a class.

Some gaps:

• I did not deal with private members in cppDefs_TBB.R as I haven't wrapped my head around cppParallelBodyClass.
• There is no testing for derivs of private members.