Add API call to reset memory selection (#4169)

* Add API to reset memory selection

Author: franzpoeschel

bindings/CXX11/adios2/cxx11/Variable.h

@@ -214,7 +214,7 @@ class Variable
      * variable.Count() = {Ny,Nx}, then memoryCount = {Ny+2,Nx+2}
      * </pre>
      */
-    void SetMemorySelection(const adios2::Box<adios2::Dims> &memorySelection);
+    void SetMemorySelection(const adios2::Box<adios2::Dims> &memorySelection = {{}, {}});
 
     /**
      * Sets a step selection modifying current startStep, countStep

source/adios2/core/VariableBase.cpp

@@ -221,6 +221,13 @@ void VariableBase::SetMemorySelection(const Box<Dims> &memorySelection)
     const Dims &memoryStart = memorySelection.first;
     const Dims &memoryCount = memorySelection.second;
 
+    if (memoryStart.empty() && memoryCount.empty())
+    {
+        m_MemoryStart.clear();
+        m_MemoryCount.clear();
+        return;
+    }
+
     if (m_SingleValue)
     {
         helper::Throw<std::invalid_argument>("Core", "VariableBase", "SetMemorySelection",