getInstrument will return a const pointer now

Refs #13758

MantidQt/MantidWidgets/inc/MantidQtMantidWidgets/SlitCalculator.h

@@ -41,7 +41,7 @@ public:
   SlitCalculator(QWidget *parent);
   virtual ~SlitCalculator();
   void setInstrument(std::string instrumentName);
-  Mantid::Geometry::Instrument_sptr getInstrument();
+  Mantid::Geometry::Instrument_const_sptr getInstrument();
   void setCurrentInstrumentName(std::string instrumentName);
   std::string getCurrentInstrumentName();
   void processInstrumentHasBeenChanged();
@@ -50,7 +50,7 @@ protected:
   Ui::SlitCalculator ui;
 
 private:
-  Mantid::Geometry::Instrument_sptr instrument;
+  Mantid::Geometry::Instrument_const_sptr instrument;
   std::string currentInstrumentName;
   void setupSlitCalculatorWithInstrumentValues(
       Mantid::Geometry::Instrument_const_sptr);

MantidQt/MantidWidgets/src/SlitCalculator.cpp

@@ -60,14 +60,14 @@ void SlitCalculator::setupSlitCalculatorWithInstrumentValues(
   const double s2sa = 1e3 * slit2Component->getDistance(*sampleComponent);
   ui.spinSlit2Sample->setValue(s2sa);
 }
-Mantid::Geometry::Instrument_sptr SlitCalculator::getInstrument() {
-  return this->instrument;
+Mantid::Geometry::Instrument_const_sptr SlitCalculator::getInstrument() {
+  return instrument;
 }
 void SlitCalculator::setCurrentInstrumentName(std::string instrumentName) {
   this->currentInstrumentName = instrumentName;
 }
 std::string SlitCalculator::getCurrentInstrumentName() {
-  return this->currentInstrumentName;
+  return currentInstrumentName;
 }
 void SlitCalculator::on_recalculate_triggered() {
   const auto currentInstrument = getInstrument();