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.

src/Base/Unit.cpp

@@ -281,11 +281,6 @@ int Unit::angle() const
     return sig.Angle;
 }
 
-bool Unit::isEmpty() const
-{
-    return Val == 0;
-}
-
 int Unit::operator [](int index) const
 {
     UnitSignature sig;
@@ -318,6 +313,11 @@ bool Unit::operator ==(const Unit& that) const
     return Val == that.Val;
 }
 
+bool Unit::operator !=(const Unit& that) const
+{
+    return Val != that.Val;
+}
+
 Unit Unit::operator *(const Unit &right) const
 {
     Unit result;
@@ -360,7 +360,7 @@ Unit Unit::operator /(const Unit &right) const
 
 std::string Unit::getString() const
 {
-    if (isEmpty()) {
+    if (Val == 0) {
         return {};
     }
 
@@ -658,6 +658,8 @@ std::string Unit::getTypeString() const
     return spec->second;
 }
 
+const Unit Unit::One                        (0, 0, 0, 0, 0, 0, 0, 0);
+
 // SI base units
 const Unit Unit::AmountOfSubstance          (0, 0, 0, 0, 0, 1);
 const Unit Unit::ElectricCurrent            (0, 0, 0, 1);