[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/Per.java

@@ -32,13 +32,14 @@ public class Per<N extends Unit<N>, D extends Unit<D>> extends Unit<Per<N, D>> {
    * Creates a new proportional unit derived from the ratio of one unit to another. Consider using
    * {@link #combine} instead of manually calling this constructor.
    *
-   * @param baseType the base type representing the unit ratio
    * @param numerator the numerator unit
    * @param denominator the denominator unit
    */
-  protected Per(Class<Per<N, D>> baseType, N numerator, D denominator) {
+  protected Per(N numerator, D denominator) {
     super(
-        baseType,
+        numerator.isBaseUnit() && denominator.isBaseUnit()
+            ? null
+            : combine(numerator.getBaseUnit(), denominator.getBaseUnit()),
         numerator.toBaseUnits(1) / denominator.toBaseUnits(1),
         numerator.name() + " per " + denominator.name(),
         numerator.symbol() + "/" + denominator.symbol());
@@ -46,6 +47,17 @@ public class Per<N extends Unit<N>, D extends Unit<D>> extends Unit<Per<N, D>> {
     m_denominator = denominator;
   }
 
+  Per(
+      Per<N, D> baseUnit,
+      UnaryFunction toBaseConverter,
+      UnaryFunction fromBaseConverter,
+      String name,
+      String symbol) {
+    super(baseUnit, toBaseConverter, fromBaseConverter, name, symbol);
+    m_numerator = baseUnit.numerator();
+    m_denominator = baseUnit.denominator();
+  }
+
   /**
    * Creates a new Per unit derived from an arbitrary numerator and time denominator units. Using a
    * denominator with a unit of time is discouraged; use {@link Velocity} instead.
@@ -63,7 +75,7 @@ public class Per<N extends Unit<N>, D extends Unit<D>> extends Unit<Per<N, D>> {
    * @param denominator the denominator for unit time
    * @return the combined unit
    */
-  @SuppressWarnings({"unchecked", "rawtypes"})
+  @SuppressWarnings("unchecked")
   public static <N extends Unit<N>, D extends Unit<D>> Per<N, D> combine(
       N numerator, D denominator) {
     final long key =
@@ -74,7 +86,7 @@ public class Per<N extends Unit<N>, D extends Unit<D>> extends Unit<Per<N, D>> {
       return existing;
     }
 
-    var newUnit = new Per<N, D>((Class) Per.class, numerator, denominator);
+    var newUnit = new Per<>(numerator, denominator);
     cache.put(key, newUnit);
     return newUnit;
   }