Fix DIV0 SEGV

.. in WPrime, and also don't bother trying to autosolve
   CP when W'bal is negative if W' is not setup
Add W'bal chart to default setup

.. when new athletes created.
Potentiall fix crash on Pickers

.. track and delete plot pickers.
Fix WPrime CP lookup SEGV

.. when zones or cp not set for the date in question

Fixes #774.
Update WPrime comment

.. to explain how we sped up the W'bal integral calculation
Fix delete SEGV

.. autodelete doesn't like detached items.
Plot Predicted W'bal in Train mode

.. only plots predicted W'bal for the workout but at least that
   means you can see if the workout is even possible.
W'bal decay to infinity

.. No more cutoff of the decay. Since we use threads its
   not such a big deal as before.

.. We now get a full, smooth decay till the end of the ride
   which was much more noticeable when predicting W'bal for
   a workout, than it was for a ride.
W'bal decay set to 1hr, infinite not practical
merge upstream into master