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/Mod/Spreadsheet/App/Cell.cpp

@@ -607,7 +607,7 @@ void Cell::setComputedUnit(const Base::Unit& unit)
     PropertySheet::AtomicPropertyChange signaller(*owner);
 
     computedUnit = unit;
-    setUsed(COMPUTED_UNIT_SET, !computedUnit.isEmpty());
+    setUsed(COMPUTED_UNIT_SET, computedUnit != Unit::One);
     setDirty();
 
     signaller.tryInvoke();
@@ -1110,7 +1110,7 @@ std::string Cell::getFormattedQuantity()
         const Base::Unit& computedUnit = floatProp->getUnit();
         qFormatted = QLocale().toString(rawVal, 'f', Base::UnitsApi::getDecimals());
         if (hasDisplayUnit) {
-            if (computedUnit.isEmpty() || computedUnit == du.unit) {
+            if (computedUnit == Unit::One || computedUnit == du.unit) {
                 QString number =
                     QLocale().toString(rawVal / duScale, 'f', Base::UnitsApi::getDecimals());
                 qFormatted = number + QString::fromStdString(" " + displayUnit.stringRep);

src/Mod/Spreadsheet/App/Sheet.cpp

@@ -802,7 +802,7 @@ void Sheet::updateProperty(CellAddress key)
                 Base::PyGILStateLocker lock;
                 setObjectProperty(key, constant->getPyValue());
             }
-            else if (!number->getUnit().isEmpty()) {
+            else if (number->getUnit() != Unit::One) {
                 setQuantityProperty(key, number->getValue(), number->getUnit());
             }
             else if (number->isInteger(&l)) {

src/Mod/Spreadsheet/Gui/SheetModel.cpp

@@ -384,7 +384,7 @@ QVariant SheetModel::data(const QModelIndex& index, int role) const
 
                 // Display locale specific decimal separator (#0003875,#0003876)
                 if (cell->getDisplayUnit(displayUnit)) {
-                    if (computedUnit.isEmpty() || computedUnit == displayUnit.unit) {
+                    if (computedUnit == Base::Unit::One || computedUnit == displayUnit.unit) {
                         QString number =
                             QLocale().toString(floatProp->getValue() / displayUnit.scaler,
                                                'f',