[wpimath] Cleanup and Enhancement of DCMotor and gearboxes

[narmstro2020]

So in an effort to clean up and enhance DCMotor and the whole concept of a gearbox I decided to do the following.

Summary of changes.

Java changes

1. DCMotor is only focused on the constants for 1 motor and doesn't even know what a reduction is.
2. The motor constants are internally stored in the Java units library. They are still public final. I questioned this, but realized that it is of no significant effort to call baseMagnitude() on any of those fields. If you are wanting those values then the units library should be pretty straightforward to navigate.
3. The static methods for each known type have been replaced with an enum called KnownDCMotor. (I'm up for better name suggestions)
4. Gearbox class that contains a DCMotor, number of motors, and a gearbox reduction.
5. Constructor overloads for Gearbox exist for combos involving 1 motor with reduction of 1. multiple motors but reduction of 1. 1 motor with reduction other than 1 and full constructor.
6. Gearbox reduction can still be changed as per the requirements of DifferentialDrive and related classes.
7. getTorque, Voltage, Current, and AngularVelocity methods for both units and non units class usages.
8. DCMotorSim renamed to GearboxSim. Simulating a simple single motor with no gearing would be handled with GearboxSim as well.
9. LinearSystemId static methods reworked. All none drivetrain sims will eventualy use one of the kv, ka methods after calculating them in their own methods and then call idenfityPosition or identifyVelocity. createDCMotorSystem replaced with createGearboxSystem.

C++ changes.
... coming soon. Currently in production.

As per usual any feedback is welcome and appreciated.

[narmstro2020]

/format

[spacey-sooty]

The static methods for each known type have been replaced with an enum called KnownDCMotor. (I'm up for better name suggestions)

Why? Factories seemed like a good approach to me