[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/test/java/edu/wpi/first/units/DistanceTest.java

@@ -12,9 +12,10 @@ import org.junit.jupiter.api.Test;
 class DistanceTest {
   @Test
   void testBaseUnitDistancePerTime() {
-    Velocity<Distance> anonBaseUnit = new Distance(1, "D", "d").per(new Time(1, "T", "t"));
+    Velocity<Distance> anonBaseUnit =
+        new Distance(null, 1, "D", "d").per(new Time(null, 1, "T", "t"));
 
-    assertTrue(BaseUnits.Velocity.equivalent(anonBaseUnit));
+    assertTrue(Units.MetersPerSecond.equivalent(anonBaseUnit));
   }
 
   @Test