Improved javadoc for ContinuousOutputModel and StepInterpolator.

The fact the arrays are reused is stated more clearly, as weel as the
fact changing the time changes the array content.

Yes, it is a bad design, we should change it in the next major version
to allocate fresh new arrays :-(
Added getInterpolatedSecondaryDerivatives to ContinuousOutputModel.

This method is a close relative to getInterpolatedDerivatives, but is
associated with the secondary state.

JIRA: MATH-1160