move isLast argument in step handler handleStep method to a separate method

[maisonobe]

The ODEStepHandler (and its field counterpart) declare two methods:

• void init(ODEStateAndDerivative initialState, double finalTime)
• void handleStep(ODEStateInterpolator interpolator, boolean isLast)

The isLast parameter in the handleStep method is set to false throughout integration except at
last step, where it is set to true.

For both simplification and symmetry with respect to the init method, this parameter should be removed from the handleStep signature and a separate void finish(ODEStateAndDerivative finalState) should be set up, with a default implementation doing nothing.