Allow loading of LTM exports to the Viewer, although the memory servers have not been started

Steps if you want to test:

• Make some memory export and store it somewhere on the FS
• Kill all memories. Only start armem core scenario
• Open memory viewer GUI, disable drop of disconnected memories
• load the export

If a memory server is running -> the imported data will be commited to the server (as before) If no memory server is running -> create a new virtual memory (only available in GUI) and show data there

source/RobotAPI/libraries/armem_gui/MemoryViewer.cpp

@@ -267,7 +267,7 @@ namespace armarx::armem::gui
         {
             std::stringstream ss;
             ss << "Memory name '" << memoryName
-               << "' is unknown. Known are: " << simox::alg::get_keys(memoryData);
+               << "' is unknown. Known are: " << simox::alg::to_string(simox::alg::get_keys(memoryData), ", ");
             statusLabel->setText(QString::fromStdString(ss.str()));
             return nullptr;
         }