Read_DGModel2D_t Subroutine

  subroutine Read_DGModel2D_t(this,fileName)
    implicit none
    class(DGModel2D_t),intent(inout) :: this
    character(*),intent(in) :: fileName
    ! Local
    integer(HID_T) :: fileId
    integer(HID_T) :: solOffset(1:3)
    integer :: firstElem
    integer :: ivar
    character(LEN=:),allocatable :: dsetName

    if(this%mesh%decomp%mpiEnabled) then
      call Open_HDF5(fileName,H5F_ACC_RDWR_F,fileId, &
                     this%mesh%decomp%mpiComm)
    else
      call Open_HDF5(fileName,H5F_ACC_RDWR_F,fileId)
    endif

    if(this%mesh%decomp%mpiEnabled) then
      firstElem = this%mesh%decomp%offsetElem(this%mesh%decomp%rankId+1)
      solOffset(1:3) = (/0,0,firstElem/)
    endif

    ! A variable whose dataset is absent keeps the value it was initialized
    ! with. This is what lets a pickup file written by an earlier version of a
    ! model - one that carried fewer solution variables - still be read: the
    ! variables the file does hold are restored, and a variable it predates
    ! retains its initial value (zero after Init, or whatever the caller set
    ! before reading). Without the check ReadArray_HDF5 would read through an
    ! invalid dataset id and dump the HDF5 error stack instead. Every rank gets
    ! the same answer for the same file, so the collective reads below stay in
    ! step.
    do ivar = 1,this%solution%nvar
      dsetName = '/controlgrid/solution/'//trim(this%solution%meta(ivar)%name)
      if(.not. DatasetExists_HDF5(fileId,dsetName)) then
        print*,__FILE__," : Pickup file holds no ",trim(dsetName), &
          " - keeping the initialized value for this variable."
        cycle
      endif
      if(this%mesh%decomp%mpiEnabled) then
        call ReadArray_HDF5(fileId,dsetName, &
                            this%solution%interior(:,:,:,ivar),solOffset)
      else
        call ReadArray_HDF5(fileId,dsetName, &
                            this%solution%interior(:,:,:,ivar))
      endif
    enddo

    call Close_HDF5(fileId)

    ! Publish the restored solution to the device. Read_DGModel1D_t has always
    ! done this; without it a GPU build restarts from whatever the device
    ! happened to hold (zeros, after Init) and silently discards the pickup
    ! file - the first device-to-host copy of the run, in CalculateEntropy or
    ! the first tendency evaluation, overwrites everything just read. This is
    ! the counterpart of the UpdateHost() that Write_DGModel2D_t performs before
    ! writing, and is a no-op on a CPU build.
    call this%solution%UpdateDevice()

  endsubroutine Read_DGModel2D_t