Wrong dimensionality of default attributes

Without setting the mesh position, gridGlobalOffset, gridSpacing attributes explicitly `openPMD-api` writes default values. This happens for example when following the First Write example. These default values are integer and not arrays with the dimension of the dataset.

See example:
```
string /data/0/meshes/dens/axisLabels attr = {"x", "y", "z"}
string /data/0/meshes/dens/dataOrder attr = "C"
string /data/0/meshes/dens/geometry attr = "cartesian"
double /data/0/meshes/dens/gridGlobalOffset attr = 0
double /data/0/meshes/dens/gridSpacing attr = 1
double /data/0/meshes/dens/gridUnitSI attr = 1
double /data/0/meshes/dens/position attr = 0
```

According to the standard:
> type: 1-dimensional array containing N (float64 / REAL8) elements, where N is the number of dimensions in the simulation

So I think the single integer is not standard compliant, even if the value is identical for all? This breaks my scipp reader (I know, #1716 is still waiting for me to have time to finish it). Now do I need to support the single integer or is this an actual inconsistency with the standard?

Using openpmd 0.16.1 and adios2 bp5 backend