fix signed vs unsigned compilation warning

Change-Id: Ib46fd747a84cfad2015d8d1a2d765f378762c26a

Chemistry/vtkAbstractElectronicData.h

@@ -35,7 +35,7 @@ public:
 
   // Description:
   // Returns the number of electrons in the molecule.
-  virtual unsigned int GetNumberOfElectrons() = 0;
+  virtual vtkIdType GetNumberOfElectrons() = 0;
 
   // Description:
   // Returns the vtkImageData for the requested molecular orbital.

Chemistry/vtkProgrammableElectronicData.h

@@ -41,8 +41,8 @@ public:
   // Description:
   // Get/Set the number of electrons in the molecule. Needed for HOMO/LUMO
   // convenience functions
-  vtkGetMacro(NumberOfElectrons, unsigned int);
-  vtkSetMacro(NumberOfElectrons, unsigned int);
+  vtkGetMacro(NumberOfElectrons, vtkIdType);
+  vtkSetMacro(NumberOfElectrons, vtkIdType);
 
   // Description:
   // Get/Set the vtkImageData for the requested molecular orbital.