Improve analysisdata reference checks.

Improve analysisdata reference checks.

The reference checking for AnalysisData objects now supports more
scenarios with multipoint data, i.e., cases where there are point sets
that do not cover all columns.

Supporting changes:
 - Add a TestReferenceChecker::checkPresent() method that allows to
   assign meaning to absence of a reference data item (and to check for
   that).
 - Split common analysisdata XML stylesheet elements to a separate xsl
   file.
 - Add a script for copying the XML stylesheets to the reference data
   directories for easier maintenance.  Master copies of shared
   stylesheets are now kept in src/testutils/.

Needed for #920, but split from the main change to keep it smaller.

Change-Id: I108c013833052c37a4f2e6c73be6161c0dae1c78