groot: complete r/w streamers

[sbinet]

PR #801 implemented a fair share of r/w values via streamers.
still, a couple of things were left for later:

• read-streamers w/ kConv (ie: converting from one builtin to another)
• r/w-streamers w/ kSkip (ie: dropping values on the floor)
• r/w-streamers w/ kSTLp (ie: a member like std::vector<T> *m_mbr;)
• r/w-streamers w/ polymorphic classes
• r/w-streamers in member-wise mode