Specify color of particles and PoissonSolver in YAML file

There is a new property "color" for particles, which can be
- a color name (e.g. "red", "blue", ...)
- a HEX code (e.g. "FF77FF")
- or the keyword "random" for random color

Also specify PoissonSolver in YAML file. Possible settings are:
- fft: use PoissonSolverFFTPeriodic()
- empty: use EmptyPoissonSolver()