Only hide model estimates when W' < 1kJ

.. instead of 10kJ
Enable selection of W'bal formula

.. Differential - uses the Clarke/Froncioni fast differential eqn

.. Integral - uses the original Skiba et al 'slow' equation
W'bal invalidate and recompute

.. when formula selection changes
Set the title when adding a chart

Fixes #967
Limit W' bal integral exp Decay to TAU * 3

.. when computing W'bal decay -- since we know the half-life
   for the decay we might as well use it rather than a pretty
   big constant of 1 hour !

.. it saves about 35% of time to compute metrics
merge upstream into master