Functionality Upgrades

In working with this tool this semester, I noticed a couple of places where the sim didn't work exactly like I wanted it to.  As next semester looms and my Winter Break to-do list begins to fill, I thought I'd knock out these easy changes while I had the time and while they were still in my head.

• You can now pause the simulation by pressing the "P" key.  This lets you pause the planet's motion in its orbit, but still toggle some of the annotation features on and off.
• The simulation speed can now be reduced as well as increased.  The sim is now configurable to run from 0.25x to 5x speed.  This allows short orbits to be slowed to a pace that is easier to observe.
• When entering the either the initial velocity or the initial distance, pressing "Enter" will restart the sim with the new data.