fix sampler reading for mono samples, and tweak planetary orbit example

[james-trayford]

some tweaks to sample reading so that the PlanetaryOrbits.ipynb example works as expected, following Issue https://github.com/james-trayford/strauss/issues/15

• only force samples to mono if they're not mono already! otherwise convert array to expected format
• use updated save routine in PlanetaryOrbits.ipynb