mrmath: Fix conflict resolution

Merge commit e467385 failed to address fact that modifications made in dae868c utilised MR::vector whereas 7d81f13 merged in #2764 reverts to use of std::vector.

cmd/mrmath.cpp

@@ -310,7 +310,7 @@ template <class Operation> class ImageKernel : public ImageKernelBase {
 
 protected:
   Voxel2Vector v2v;
-  vector<Operation> data;
+  std::vector<Operation> data;
 };
 
 void run() {