Update One_particle_Test.yaml file by output.
Clear files in the correct dirctory.
Specify color of particles in YAML file

There is a new property "color", which can be
- a color name (e.g. "red", "blue", ...)
- a HEX code (e.g. "FF77FF")
- or the keyword "random" for random color
Specify PoissonSolver in YAML file.

Possible settings are:
- fft: use PoissonSolverFFTPeriodic()
- empty: use EmptyPoissonSolver()
Merge branch 'upstream' into file

Conflicts:
	pixi/input/One_particle_Test.yaml
	pixi/input/Pair_of_particles.yaml
	pixi/input/Two_stream_instability.yaml
Merge pull request #89 from aipp/file

Specify color of particles and PoissonSolver in YAML file