[wpiunits] Change units to track their base unit, instead of their base class (#6342)

Unit objects now have a reference to the base unit from which they're derived. Constructing a unit object without specifying a base unit implicitly signifies that it's its own base unit, eg new Angle(null, 1, "Radian", "rad") would be the base angle unit of radians, while new Angle(Radians, 2 * PI, "Rotation", "R") would be a new angle unit based on radians.

This fixes much of the hacky code surrounding the derived unit types Velocity, Per, and Mult, but is a breaking change for any user code that defines custom unit classes or uses the anonymous unit type.

wpiunits/src/main/java/edu/wpi/first/units/Temperature.java

@@ -15,7 +15,11 @@ package edu.wpi.first.units;
  */
 public class Temperature extends Unit<Temperature> {
   Temperature(
-      UnaryFunction toBaseConverter, UnaryFunction fromBaseConverter, String name, String symbol) {
-    super(Temperature.class, toBaseConverter, fromBaseConverter, name, symbol);
+      Temperature baseUnit,
+      UnaryFunction toBaseConverter,
+      UnaryFunction fromBaseConverter,
+      String name,
+      String symbol) {
+    super(baseUnit, toBaseConverter, fromBaseConverter, name, symbol);
   }
 }