Merge pull request #3122 from pnorbert/docs-whatsnew

    BeginStep/EndStep calls are *NOT allowed*. SetStepSelection() can be used to access specific steps in the file. 

  * **adios2::Mode::Read** mode now requires using BeginStep/EndStep loop to access steps serially one by one. Variable inquiry 

    fails outside BeginStep/EndStep sections. You need to modify your Open() statement to use the random-access mode if your

    code is accessing all steps in an existing file.

    code wants to access all steps in any order in an existing file.

  * **adios2::ADIOS::EnterComputationBlock()**, **adios2::ADIOS::ExitComputationBlock()** are hints to ADIOS that a process is in a computing (i.e. non-communicating) phase. BP5 asynchronous I/O operations can schedule writing during such phases to avoid interfering with the application's own communication. 

  * GPU-aware I/O supports passing device-memory data pointers to the ADIOS2 Put() function, and ADIOS2 will automatically download data from the device during I/O. Alternatively, an extra member function of the Variable class, **SetMemorySpace(const adios2::MemorySpace mem)** can explicitly tell ADIOS2 whether the pointer points to device memory or host memory.

  * GPU-aware I/O supports passing device-memory data pointers to the ADIOS2 `Put()/Get()` functions, and ADIOS2 will automatically download/upload data from/to the device during I/O. Alternatively, an extra member function of the Variable class, **SetMemorySpace(const adios2::MemorySpace mem)** can explicitly tell ADIOS2 whether the pointer points to device memory or host memory.

New features

   * BP5 data format and engine. This new engine optimizes for many variables and many steps at large scale. 

   * **BP5** data format and engine. This new engine optimizes for many variables and many steps at large scale. 

     It is also more memory efficient than previous engines, see :ref:`BP5`. 

   * Plugin architecture to support external *engines* and *operators* outside the ADIOS2 installation, see :ref:`Plugins` 

   * GPU-Aware I/O for writing data from device memory, using CUDA (NVidia GPUs only), see :ref:`GPU-aware I/O`

   * **Plugin** architecture to support external *engines* and *operators* outside the ADIOS2 installation, see :ref:`Plugins` 

   * **GPU-Aware I/O** for reading/writing data to/from device memory, using CUDA (NVidia GPUs only), see :ref:`GPU-aware I/O`

Other changes