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.