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/Quantity.cpp

@@ -270,10 +270,10 @@ std::string Quantity::getSafeUserString() const
     return Tools::escapeQuotesFromString(userStr);
 }
 
-/// true if it has a number without a unit
+/// true if unit equals to 1, therefore quantity has no dimension
 bool Quantity::isDimensionless() const
 {
-    return myUnit.isEmpty();
+    return myUnit == Unit::One;
 }
 
 /// true if it has a specific unit or no dimension.