Solving unit equivalence

[andi-huber]

Might be a requirement to solve the unit equivalence problem ...
unitsofmeasurement/unit-api#89

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This change is 

[andi-huber]

In the meantime I reworked the way identity transformations are handled when composing UnitConverters. Also this is a first step of solving:
#3

[andi-huber]

With PrefixTest we have 2 disabled tests, testing for equality. We can show now that testing for equivalence does work:

  @Test
  public void testEquivalence() {
	  ComparableUnit a = (ComparableUnit) MICRO(GRAM); 
	  ComparableUnit b = (ComparableUnit) GRAM.divide(1_000_000);
	  assertEquals(true, a.isEquivalentOf(b));
  }

  @Test
  public void testNestedEquivalence() {
	  ComparableUnit a = (ComparableUnit) MICRO(GRAM);
	  ComparableUnit b = (ComparableUnit) GRAM.divide(1000).divide(1000);
	  assertEquals(true, a.isEquivalentOf(b));
  }

[andi-huber]

TODO
Fix failing CompositionEquivalenceTest tests by completing

• AbstractUnit.Simplifier TODOs
• any concrete UnitConverters that do not yet fully implement AbstractConverter's

	/**
	 * Guard for {@link #simpleCompose(AbstractConverter)}
	 * @param that
	 * @return whether or not a 'simple' composition of transformations is possible
	 */
	protected abstract boolean isSimpleCompositionWith(AbstractConverter that);
	
	/**
	 * Guarded by {@link #isSimpleCompositionWith(AbstractConverter)}
	 * @param that
	 * @return a new AbstractConverter that adds no additional conversion step
	 */
	protected AbstractConverter simpleCompose(AbstractConverter that) {
		throw new IllegalStateException(
				String.format("Concrete UnitConverter '%s' does not implement simpleCompose(...).", this)); 
	}