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.
Merge pull request #77 from aipp/electricfield

Add new panel to display the electric field