Minimum CP for W'bal going negative

.. on ride plot, when W'bal is plotted we put a mesage to say
   that CP is wrong, and autosolve for W'bal to zero.

.. this patch also optimises WPrime to make it a member of the
   ridefile class and this always available (even for intervals).

.. fixed a SEGV when computing metrics for intervals by shifting
   the timestamps to always start from zero.
Fix DIV/0 SEGV with WPrime
Fix temperature curve

Cosmetic fixups to make sure temperature is visible
and the fill is honoured.
Fix W'bal calculation

.. the optimisation was a little eager and took out
   the integration step !

.. access to PCP is now hidden behind a function so
   it is only computed when neccessary.

.. the calculation is still significantly faster than
   the old one but not as instant as the non-integrating
   version (but then it wasn't working either!).
merge upstream into master