Two more span related bugs

Created by: germasch

I know it may look like I'm just poking holes in the code. But actually, I spent some time earlier implementing my proposed solution for multi-block buffered I/O and that's just what I saw along the way.

In #1293, I added another commit, which adds three tests for the current span stuff. The first one works. The 2nd one actually fails immediately:

[ RUN      ] ADIOS2_CXX11_API_IO.MultiBlockPutZeroCopySync2
unknown file: Failure
C++ exception with description "ERROR: units 50 not supported in call to BytesFactor
" thrown in the test body.
[  FAILED  ] ADIOS2_CXX11_API_IO.MultiBlockPutZeroCopySync2 (1 ms)

The third one may now be fixed with @williamfgc's latest patch, but for now I get this as output:

[kai@macbook build (pr/bug)]$ bin/bpls -dD multi0_sync3.bp
  double   var   {10}
        step 0:
          block 0: [0:4]
    (0)    5 6 7 8 9
          block 1: [0:4]
    (0)    5 6 7 8 9