[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/UnitsTest.java

@@ -135,7 +135,7 @@ class UnitsTest {
     assertEquals(9.80665, Gs.of(1).in(MetersPerSecondPerSecond), thresh);
     assertEquals("G", Gs.name());
     assertEquals("G", Gs.symbol());
-    assertEquals(Units.AnonymousBaseUnit, Gs.getUnit());
+    assertEquals(MetersPerSecond, Gs.getUnit());
     assertEquals(Seconds, Gs.getPeriod());
   }