Vector values now visible.

After consulting with other physics instructors and getting feedback from others who have interacted with the demo, we've included some new features and fixed some bugs.

New Features:

• The visibility of the various vectors can be toggled on and off using keyboard shortcuts now in addition to using the "Vectors On/Off" button.  Pressing "v" will turn the vectors on or off.
• The visibility of the force vector and data can be toggled on and off separately by pressing the "f" key.  The force vector on the ship will still only display if all the vectors are turned on.  
• All vectors now in addition to their visual representations on the ship will also have their numeric values shown next to the legend in the top left of the screen.  

Bug Fixes:

• The velocity magnitude is now capped at 1000.0 m/s to prevent overflow issues.
• Both the mass and the thrust values are clamped between the values of 1.0 and 1000.0 to prevent overflow and division-by-zero errors.
• The mass and thrust input fields are restricted to take only numeric input.

As with any of my demos, this is meant to be a useful and productive classroom tool.  If you see any way to improve this demo's utility in the classroom or if you find bugs, please leave me a comment or send me an email.