Throw explanatory message rather than abort.
Added point processor for multi-table output.

The point processor just looks at nodes to determine the output -
we should have another one which is able to do centroids for
matching types.

Added a test case for this and factored out a bunch of common code
for setting up test databases.
Merge remote-tracking branch 'MapQuest/kk_multiback' into mla_multi_backend
Store and use multiple outputs.
Add (really unimaginative) test for multi-table output.
fix a bunch of warnings. reinstate a controllable buffer
Merge remote-tracking branch 'origin/mla_multi_backend' into kk_multiback