Move panels to new package ui.panel.

This includes:
- Particle2DPanel
- PhaseSpacePanel
Add ElectricFieldPanel.

This panel shows the one-dimensional electric field along the x-direction.
Several field lines for various grid positions along the y-direction are
superimposed.
Change panels by right-clicking on them.
Add new tab with text area.
add three buttons for Open, Save and Apply

Open and Save open a file dialog, but so far without any functionality.
Open file and write content into text field.
Parse the input string and reset the simulation.
Implement basic YAML parser (using SnakeYAML).
Write initialization text area to file.
Add confirmation dialog if file exists.
Merge pull request #76 from aipp/graphicspanel

Add second panel for graphical display of phase space
Merge pull request #77 from aipp/electricfield

Add new panel to display the electric field
Initial conditions are now largely independent off the chosen grid step, this was achieved by considering charge density instead of charge in the FFT routine by getting sure that each charge is treated like a uniformly charged rectangle of the size of an elementary cell. For this purpose measurement of snapshot files has been added, now we can do 3D plots of some grid quantities.
Merge with master, minor manual changes were needed.

Conflicts:
	pixi/src/main/java/org/openpixi/pixi/ui/Particle2DPanel.java
Start file chooser in "input" subdirectory.
New YAML setting for pair of particles.

Add YamlParticle class to create new particles from a YAML description.
New YAML settings for two stream instability.

Add YamlParticleStream class to create stream of particles.
automatically apply settings after opening a new file.
Merge pull request #78 from dgelfand/master

Initial conditions grid step independent.
Small cosmetic changes

- remove unnecessary import
- change order of setting particle properties and adding it to
  the list of particles (the order does not really matter)