Add a demo on pseudoheating

[henry2004y]

https://doi.org/10.1063/1.4789608
https://doi.org/10.1063/1.4863833

The idea is that particle motions guided by waves may generate pseudoheating of the distribution function if the spatial resolution is low. This is an interesting result and not difficult to reproduce.

An even more interesting investigation is that whether the conclusion will change in a self-consistent PIC simulation?