Const Records: Relax shape for Particles

For advanced, highly parallel I/O output, we developed a new
method in ADIOS2 that does not need an initial metadata gather
("JoinedArrays"). To be able to use this mode, we need to relax
the requirements to write a shape for constant records in a species
(particle group), because otherwise we still have to do a collective
gather.

This adds the need for a slight additional read fallback implementation
on the reader side.

Author: ax3l

STANDARD.md

@@ -731,8 +731,10 @@ The record's *data set* `<componentName>` must be replaced with an empty
 
 `shape` is a 1-dimensional array of `N` *(uint64)* elements, where `N` is the
 number of dimensions of the record. It contains the number of elements of each
-dimension that are replaced with a constant value. For `mesh` based records,
-the order of the `N` values must be identical to the axes in `axisLabels`.
+dimension that are replaced with a constant value.
+  - For `mesh` based records, the order of the `N` values must be identical to the axes in `axisLabels`.
+  - For `particle` records, the `shape` attribute can be skipped if there is *at least one more record* in the same particle group (species) that is non-constant or is constant and has the `shape` attribute.
+    Read-logic will then pick any other record to recover the `shape`.
 
 Other required attributes that where previously stored on the *data set* need
 to be added to the new sub-group as well.