Base: improve Quantity parser

Add Quantity::concat() to handle quantity concatenation, which can
either be plus or minus depending on the signedness of the left
quantity.

1m1mm is interpreted as 1m plus 1mm,

-1m1mm means -1m minus 1mm.

-1m-1mm means -1m minus -1mm, i.e. -1m plus 1mm.

(0-1)m-1mm means -1m minus -1mm, i.e. -1m plus 1mm.

src/Base/Quantity.cpp

@@ -164,6 +164,16 @@ Quantity Quantity::pow(double p) const
         );
 }
 
+Quantity Quantity::concat(const Quantity &p) const
+{
+    if (this->_Unit != p._Unit)
+        throw Base::UnitsMismatchError("Quantity: Unit mismatch in concat operation");
+    if(_Value >= 0.0)
+        return Quantity(this->_Value + p._Value,this->_Unit);
+    else
+        return Quantity(this->_Value - p._Value,this->_Unit);
+}
+
 Quantity Quantity::operator +(const Quantity &p) const
 {
     if (this->_Unit != p._Unit)

src/Base/Quantity.h

@@ -137,6 +137,7 @@ class BaseExport Quantity
     Quantity& operator =(const Quantity&);
     Quantity pow(const Quantity&)const;
     Quantity pow(double)const;
+    Quantity concat(const Quantity &)const;
     //@}
 
     const QuantityFormat& getFormat() const {