[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/VelocityTest.java

@@ -20,8 +20,8 @@ import org.junit.jupiter.api.Test;
 class VelocityTest {
   @Test
   void testBaseUnit() {
-    assertTrue(MetersPerSecond.equivalent(BaseUnits.Velocity));
-    assertTrue(Meters.per(Second).equivalent(BaseUnits.Velocity));
+    assertTrue(MetersPerSecond.equivalent(MetersPerSecond));
+    assertTrue(Meters.per(Second).equivalent(MetersPerSecond));
   }
 
   @Test