Added RowLocation class.

- Added RowLocation class for storing paths to nodes in the BatchWidget.
- Added description as a concept in new dev-docs page for BatchWidget.

Re #22263

dev-docs/source/BatchWidgetInternals.rst

+.. _BatchWidget:
+
+=============================
+Batch Widget Developer Manual
+=============================
+
+.. contents:: Contents
+   :local:
+
+The Batch Widget is a hierarchical grid-based widget designed for interfaces which need a
+spreadsheet-like interface for configuring and indicating the status of multiple (batch) reduction
+jobs.
+
+
+========
+Concepts
+========
+
+Row Location
+^^^^^^^^^^^^
+
+A row location is the notion of an individual table row's position within the table. Since the
+table is unlike a traditional spreadsheet in that it is hierarchical the table is more like a tree
+of rows where all non-leaf nodes can have any number of children.
+
+In practice some interfaces, such as Reflectometry are likely to want to constrain the
+number of children or depth of the tree, and the batch widget has mechanisms for performing this.
+
+Currently a row location is represented as a path from the root node to the row node in question,
+this is actualised in the code as an ordered list of integers where each element represents the index
+of the next node in the path relative to it's predecessor in the path. Some example nodes, their
+corresponding paths, and their representations are illustrated in the diagram below.

qt/widgets/common/CMakeLists.txt

@@ -77,6 +77,7 @@ set ( SRC_FILES
 	src/DataProcessorUI/QDataProcessorWidget.cpp
 	src/DataProcessorUI/QtDataProcessorOptionsDialog.cpp
 	src/DataProcessorUI/VectorString.cpp
+  src/Batch/RowLocation.cpp
 	src/DataSelector.cpp
 	src/DiagResults.cpp
 	src/DoubleSpinBox.cpp
@@ -176,6 +177,7 @@ set ( MOC_FILES
   inc/MantidQtWidgets/Common/SelectionNotificationService.h
   inc/MantidQtWidgets/Common/AlgorithmSelectorWidget.h
   inc/MantidQtWidgets/Common/CheckboxHeader.h
+  inc/MantidQtWidgets/Common/Batch/RowLocation.h
   inc/MantidQtWidgets/Common/DataProcessorUI/AbstractTreeModel.h
   inc/MantidQtWidgets/Common/DataProcessorUI/QtCommandAdapter.h
   inc/MantidQtWidgets/Common/DataProcessorUI/GenericDataProcessorPresenter.h

qt/widgets/common/inc/MantidQtWidgets/Common/Batch/RowLocation.h

+#ifndef MANTIDQTMANTIDWIDGETS_ROWLOCATION_H_
+#define MANTIDQTMANTIDWIDGETS_ROWLOCATION_H_
+#include <vector>
+#include "MantidQtWidgets/Common/DllOption.h"
+namespace MantidQt {
+namespace MantidWidgets {
+namespace Batch {
+
+using RowPath = std::vector<int>;
+
+class EXPORT_OPT_MANTIDQT_COMMON RowLocation {
+public:
+  RowLocation(RowPath path);
+  RowPath const &path() const;
+  int rowRelativeToParent() const;
+  bool isRoot() const;
+
+private:
+  RowPath m_path;
+};
+}
+}
+}
+#endif

qt/widgets/common/src/Batch/RowLocation.cpp

+#include "MantidQtWidgets/Common/Batch/RowLocation.h"
+namespace MantidQt {
+namespace MantidWidgets {
+namespace Batch {
+
+RowLocation::RowLocation(RowPath path) : m_path(std::move(path)) {}
+RowPath const &RowLocation::path() const { return m_path; }
+int RowLocation::rowRelativeToParent() const { return m_path.back(); }
+bool RowLocation::isRoot() const { return m_path.empty(); }
+
+}
+}
+}