odeint - solving ordinary differential equations in c++ v2
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friedrichatgc and mariomulansky Enable the adaption of the maximal step size of dense output steppers. (
#225)

* Enable the adaption of the maximal step size of dense output steppers.

For efficient simulation of "hybrid" systems the integrator must approach
the sample points where the discrete variables change their value.

(hybrid systems = systems of ODEs which include discrete variables, beeing
internal variables of the system which only change their value at discrete
sample points)

Approaching sample points can be done by adapting the maximal integrator
step size to min(max_step_size, next_sample_point_time - current_time)
before each do_step.

To achive this in odeint for all dense output steppers the following
changes must be done (which does not change the existing API):
- make private members in bulirsch_stoer_dense_out,
  default_step_adjuster, rosenbrock4_controller protected.
- allow std::ref/boost::ref for step_adjuster in controlled_runge_kutta
  and controlled_runge_kutta and for stepper in rosenbrock4_dense_output
  by unwrapping these before use.
This allows to pass the step adjusters by reference to the dense output
steppers which than allows to change the maximal step size (in the step
adjuster) before each call to do_step.

* Added test for a reference controller in the Rosenbrock4 dense output stepper.

* Make in bulirsch_stoer_dense_out only the required m_max_dt member
protected not all.

Extend the test in rosenbrock4.cpp to test that the controller is a
reference and the maximal step size is applied.

* Fixed build with gcc-4.8
Latest commit 5dd9519 Feb 13, 2018

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odeint is a highly flexible library for solving ordinary differential equations.