Bring ParameterGroup interface into scope.

The constructor accepts a parameter::ParameterGroup reference and thus
needs a valid interface in scope.  Relying on header pollution is
unwise.
Fix DisjointColumn test case

The test started failing in commit 7d7f62e, but this was not detected
due to no automatic test environment.  The commit changed the cell
numbering from "per-column (K,I,J)-ordering" to "per-plane
(I,J,K)-ordering".  Consequently, the "correct_answer" seized to be
correct.

This change restores the "correct_answer" in the ordering introduced by
commit 7d7f62e.

While here, adjust style of the DisjointColumn test case for legibility.
Merge pull request #134 from bska/release-cleanup

Assorted clean-ups in preparation of release