Refactored get accessor for gasLevel

Generally, the convention for creating public getters for private
instance variables is to name them as follows:
```
//…
private double someVar;
public double getSomeVar() {
  return this.someVar;
}
//…
```
"xx gallons of gas left" report now uses getGasLevel() accessor

Since the Car class knows its own gas level, and provides a public get
accessor for it, there isn’t a reason (that I can think of) to locally
compute the gas level.
getGasLevel() now guards against negative numbers
drive() is now a more-accurate simulation

I expanded the drive() method of the Car class to model a couple of
things:
- Maximum distance possible
- Distance the driver intends to drive
- Actual distance driven

Additionally, I added range checking: maximumDistance and
intendedDistance can’t be less than zero; actualDistance can’t be
greater than maximumDistance.

Finally, drive() now returns the actualDistance (actual distance
driven), as a double.
Added output showing number of miles actually driven

Also notice that the (previously named) milesDriven variable has been
refactored to “milesToDrive”, while a new “milesDriven” variable has
been created to hold the actual miles driven.
Updated to account for renamed gasLevel accessor
Removed .class files
Updated comments