message: Read VDB grid names and select the first for import
  Fixes issue when "density" isn't a valid grid.
diff: -// Copyright 2009-2020 Intel Corporation
  +// Copyright 2009-2021 Intel Corporation
  // SPDX-License-Identifier: Apache-2.0
  #include "Vdb.h"
  @@ -143,7 +143,24 @@ namespace ospray {
  openvdb::io::File file(fileNameAbs.c_str());
  std::cout << "loading " << fileNameAbs << std::endl;
  file.open();
  - grid = file.readGrid("density");
  +
  + std::vector<std::string> gridNames;
  +
  + // Locate valid grid names within the file
  + for (auto nameIter = file.beginName(); nameIter != file.endName();
  + ++nameIter)
  + gridNames.push_back(nameIter.gridName());
  +
  + if (gridNames.size() > 1) {
  + std::cout << " multiple grids" << std::endl;
  + for (auto &n : gridNames)
  + std::cout << " : " << n << std::endl;
  + }
  +
  + // XXX at some point, add the ability to select which grid(s). For now,
  + // just choose the first.
  + std::cout << " loading grid: " << gridNames[0] << std::endl;
  + grid = file.readGrid(gridNames[0]);
  file.close();
  } catch (const std::exception &e) {
  const std::string err = (std::string("Error loading ")
  