Display of `nanotime` types incorrect in Jupyter Notebook

[lsilvest]

In Jupyter Notebook, the nanotime types do not display correctly except for nanoperiod. See what we can do about that...

[eddelbuettel]

Hm. They are all S4 classes in some cases (nanoperiod) it dispatches correctly and in the others it doesn't? Odd.

[eddelbuettel]

That still seems unchanged per a quick test in notebook.