example notebook doesn't seem to support display_units

[zbeekman]

All the examples in the jupyter/iPython notebook where the output is supposed to be in a different unit, all appear to be in the original unit, not the desired unit. I'm not sure if this is a problem that is unique to the notebook (maybe it needs updating) or if it is a problem with the natu library

[zbeekman]

See also #23 and #24