refs #8794 Replaced vector by std::pair structure

I am unsure about it, as the functional equivalent of this function for ConvertToMD methods would return more then 2 values. And some functional modifications of this method may also return more then 2 values. But after some thinking decide to make the change for the time being and then see if expansion is necessary.

Code/Mantid/Framework/Kernel/inc/MantidKernel/Unit.h

@@ -167,7 +167,7 @@ class MANTID_KERNEL_DLL Unit
   virtual double conversionTOFMax()const=0;
 
   /**The range where conversion to TOF from given units is monotonic and reversible*/
-  virtual std::vector<double> conversionRange()const;
+  virtual std::pair<double,double> conversionRange()const;
 
 protected:
   // Add a 'quick conversion' for a unit pair

Code/Mantid/Framework/Kernel/src/Unit.cpp

@@ -161,14 +161,12 @@ double Unit::convertSingleFromTOF(const double xvalue, const double& l1, const d
   return this->singleFromTOF(xvalue);
 }
 
-std::vector<double> Unit::conversionRange()const
+std::pair<double,double> Unit::conversionRange()const
 {
   double u1=this->singleFromTOF(this->conversionTOFMin());
   double u2=this->singleFromTOF(this->conversionTOFMax());
-  std::vector<double> range(2);
-  range[0] = std::min(u1,u2);
-  range[1] = std::max(u1,u2);
-  return range;
+  //
+  return std::pair<double,double>(std::min(u1,u2),std::max(u1,u2));
 }
 
 namespace Units

Code/Mantid/Framework/Kernel/test/UnitTest.h

@@ -36,8 +36,8 @@ std::string convert_units_check_range(const Unit &aUnit,std::vector<double> &sam
 
 
   auto range = aUnit.conversionRange();
-  double tof1=aUnit.singleToTOF(range[0]);
-  double tof2=aUnit.singleToTOF(range[1]);
+  double tof1=aUnit.singleToTOF(range.first);
+  double tof2=aUnit.singleToTOF(range.second);
   bool t_increases(true);
   if (tof1>tof2)
     t_increases=false;
@@ -61,16 +61,16 @@ std::string convert_units_check_range(const Unit &aUnit,std::vector<double> &sam
 
   const size_t nSteps(100);
 
-  double step = (range[1]-range[0])/nSteps;
+  double step = (range.second-range.first)/nSteps;
   if (step == std::numeric_limits<double>::infinity())
   {
     step =(DBL_MAX/nSteps)*2;
   }
 
-  double t1 = aUnit.singleToTOF(range[0]);
+  double t1 = aUnit.singleToTOF(range.first);
   for(size_t i=1;i<=nSteps;i++)
   {
-    double unitVal=range[0]+double(i)*step;
+    double unitVal=range.first+double(i)*step;
     double tofVal = aUnit.singleToTOF(unitVal);
     if (t_increases)
     {