Workaround for IBM compiler in data_out_base.cc

This is a workaround for the IBM XL compiler as suggested by Kevin
Drzycimski on the mailing list.
Merge pull request #171 from tamiko/workaround_for_ibm

Workaround for IBM compiler in data_out_base.cc
Augment documentation motivated by Martin's post on the mailing list.
Merge pull request #172 from bangerth/doc-fixes-9

Augment documentation motivated by Martin's post on the mailing list.