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.
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.
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)
Revert "Initial conditions grid step independent."
Merge pull request #80 from openpixi/revert-78-master

Revert "Initial conditions grid step independent."
Bug fix in the XML parser: set simulation properties correctly.

Set simulation width and height instead of timestep.
EmptyPoissonSolver added. New initial conditions for testing purposes added.
Merge pull request #79 from aipp/file

Load initial settings from a YAML file
Merge pull request #81 from aipp/bugfix

Bug fix in the XML parser: set simulation properties correctly.
Revert "Revert "Initial conditions grid step independent."" Now the Empty PoissonSolver is included, so the new version should compile without problems.

This reverts commit 0a1cb7742cd1be2ecd04973b26d2cdc96494d393.
Merge changes made by Andreas together with a reverted revert into my master branch.