Fixed missing candidate summary report from model

The CandidateSessionSummaryReport was not being added to the MVC
Model when required in the non-LTI instructor MVC layer. Fixed here.
Fixed regression introduced in 5e297fb

The new logic failed to correctly account for the possibility
of there being different variables in different sessions, which
can occur if the assessment package is replaced at some point.

Missing variables now generate a 'N/A' result in the delivery
reports.
Fixed ordering in sample assessment JPA query

We are now ordering by aid, which is the best way of getting
insertion order here as the main alternative (creationTime) does not
work here since all the samples are loading with the same timestamp.
Minor fix to GU-Formulae-006-BMI.xml example

Some spaces were missing in the itemBody, making it look a bit odd.
Fix to XSLT generating testPart navigation

There was a left-over import in here which should have been removed
during earlier refactoring. This was causing rubricBlocks to come
through as-is, instead of being converted to divs.
Fixed some minor validn issues in rendering XSLT

The buttons in tests had divs inside spans, which of course isn't valid.
So I've replaced these with spans.
Some minor tweaks and tidying to rendering CSS

I've decreased the font size slightly to allow for more comfortable
embeddings into frames (e.g. by LTI). Also some minor tweaks to
spacing within the rendered page.
Fixed lookup of duration variable in tests

The ResolvedAssessmentTest was previously ignoring the built-in
duration variable. This is now fixed.

(Note however that we do not yet support the more general case of
looking up the duration of individual testParts or assessmentSections.)
Temporary tweak to rendering Cache-Control headers

I have been experiencing some cache oddness this week, so have
temporarily tweaked and simplified the HTTP headers somewhat to
safer values while I find a better long term solution.
Minor tweak to 'end test part' event ordering

This now ensures that the duration timers have been updated
before outcome processing starts, just in case OP wants to refer
to duration variables. This fixes issue #42.