Handle append scalar in CompositeDataSetAttributes.

DataSetAttributes handles appending a scalar by creating a n-tuple array
from that scalar. CompositeDataSetAttributes needed to do the same. This
is essential to get operations like computing max(VAR) and then adding
that as point or cell data on composite dataset to work.

Change-Id: Iedb7d400fb2104073b0dbe11dd386afc81fcec69
Merge topic 'fix_calculator' into master

de46ebbd Handle append scalar in CompositeDataSetAttributes.