Base: Units: introduce unit one

Dimensionless quantities have all exponents equal to zero.
Such quantities are simply numbers. The associated unit is
the unit one, symbol 1, although this is rarely explicitly
written.

See chapter 2.3.3 Dimensions of quantities, The International
System of Units, 9th edition.

tests/src/Base/Unit.cpp

@@ -121,7 +121,7 @@ TEST(Unit, TestPowNoDim)
 {
     Base::Unit unit {};
     EXPECT_EQ(unit.pow(2), Base::Unit {0});
-    EXPECT_EQ(unit.isEmpty(), true);
+    EXPECT_EQ(unit == Base::Unit::One, true);
 }
 
 TEST(Unit, TestPowEQ1)