Aggregation + Append causes corrupt data

[pnorbert]

Describe the bug
Writing to an existing BP file with Append mode only works correctly if there is no aggregation (in either step).
If there is aggregation, the appended step's data will be corrupt (even if the same aggregation ratio is used).

To Reproduce

1. in testing/adios2/engine/bp/TestBPWriteAppendReadADIOS2.cpp, lines 173 and 324 (after io.AddTransport)

• add io.SetParameter("SubStreams", "1");

2. run the test in build directory:

$ mpirun -n 4 bin/Test.Engine.BP.WriteAppendReadADIOS2.MPI --gtest_filter=BPWriteAppendReadTestADIOS2.ADIOS2BPWriteAppendRead2D2x4 BP4

3. bpls shows that the second batch of steps (3..5) are corrupt

$ ./bin/bpls -la ADIOS2BPWriteAppendRead2D2x4Test.bp -d i16 -n 16
  int16_t         i16            6*{2, 16} = -509 / 543
    (0,0, 0)    513 514 -509 516 514 515 -508 517 515 516 -507 518 516 517 -506 519
    (0,1, 0)    -507 518 -505 520 -506 519 -504 521 -505 520 -503 522 -504 521 -502 523
    (1,0, 0)    517 518 -505 520 518 519 -504 521 519 520 -503 522 520 521 -502 523
    (1,1, 0)    -503 522 -501 524 -502 523 -500 525 -501 524 -499 526 -500 525 -498 527
    (2,0, 0)    521 522 -501 524 522 523 -500 525 523 524 -499 526 524 525 -498 527
    (2,1, 0)    -499 526 -497 528 -498 527 -496 529 -497 528 -495 530 -496 529 -494 531
    (3,0, 0)    525 526 -497 528 622 0 0 0 527 528 -495 530 622 0 0 0
    (3,1, 0)    -495 530 -493 532 28160 0 0 0 -493 532 -491 534 28160 0 0 0
    (4,0, 0)    529 530 -493 532 0 0 0 28160 531 532 -491 534 0 0 0 28160
    (4,1, 0)    -491 534 -489 536 0 0 0 0 -489 536 -487 538 0 0 0 0
    (5,0, 0)    533 534 -489 536 0 0 28160 0 535 536 -487 538 0 0 28160 0
    (5,1, 0)    -487 538 -485 540 0 0 0 1134 -485 540 -483 542 0 0 0 1134