Re #21181. MPI support for ExtractSingleSpectrum.

Framework/Algorithms/inc/MantidAlgorithms/ExtractSingleSpectrum.h

 #ifndef MANTID_ALGORITHMS_EXTRACTSINGLESPECTRUM_H_
 #define MANTID_ALGORITHMS_EXTRACTSINGLESPECTRUM_H_
 
-//----------------------------------------------------------------------
-// Includes
-//----------------------------------------------------------------------
-#include "MantidAPI/Algorithm.h"
+#include "MantidAPI/ParallelAlgorithm.h"
 
 namespace Mantid {
 namespace Algorithms {
@@ -43,7 +40,7 @@ namespace Algorithms {
     File change history is stored at: <https://github.com/mantidproject/mantid>
     Code Documentation is available at: <http://doxygen.mantidproject.org>
 */
-class DLLExport ExtractSingleSpectrum : public API::Algorithm {
+class DLLExport ExtractSingleSpectrum : public API::ParallelAlgorithm {
 public:
   /// Algorithm's name
   const std::string name() const override { return "ExtractSingleSpectrum"; }

Framework/Algorithms/src/ExtractSingleSpectrum.cpp

-//----------------------------------------------------------------------
-// Includes
-//----------------------------------------------------------------------
 #include "MantidAlgorithms/ExtractSingleSpectrum.h"
 #include "MantidAPI/MatrixWorkspace.h"
 #include "MantidAPI/TextAxis.h"

docs/source/development/AlgorithmMPISupport.rst

@@ -462,6 +462,7 @@ CropToComponent                   all
 CropWorkspace                     all             see ExtractSpectra regarding X cropping
 ExtractSpectra2                   all             currently not available via algorithm factory or Python
 ExtractSpectra                    all             not supported with ``DetectorList``, cropping in X may exhibit inconsistent behavior in case spectra have common boundaries within some ranks but not within all ranks or across ranks
+ExtractSingleSpectrum             all             in practice ``ExecutionMode::Distributed`` not supported due to current nonzero-spectrum-count limitation
 FilterBadPulses                   all
 FilterByLogValue                  all
 FilterByTime                      all