Fixed sensitivity calculation when using CVodesIntegrator::step

Previously, the sensitivity coefficient matrix m_yS was being updated only when
the `integrate` method was used to advance the system. Now, the sensitivities
are updated when a coefficient is requested after taking a timestep, regardless
of the method used to advance the system.

Cherry picked from trunk r1884

git-svn-id: https://cantera.googlecode.com/svn/cantera/branches/2.0@2280 02a645c2-efd0-11dd-984d-ab748d24aa7e