bpls2 + Global Value over many steps: min/max incorrect
*Created by: pnorbert*
Global (one per application) value written every step: the min and max is undetermined, or equals to the value in the first step. It should be calculated from the values across the steps when reading from a file.
See variable __Step__ from example __values_write__:
```
ADIOS2/build$ mpirun -n 4 ./bin/values_write
ADIOS2/build$ ./bin/bpls2 values.bp -l
unsigned int Nparts 5*scalar = 0 / 6
int Nproc scalar = 4
int ProcessID scalar = 0
int Step 5*scalar = 0 / 0
ADIOS2/build$ ./bin/bpls2 values.bp -lDd Step
int Step 5*scalar = 0 / 0
step 0: 1 instances available
0
step 1: 1 instances available
1
step 2: 1 instances available
2
step 3: 1 instances available
3
step 4: 1 instances available
4
```
The min/max should be 0/5 in this example.
issue