value_type cannot have an explicit contructor

[rabraker]

There are many places in odeint similar to the following (taken from runge_kutta_dopri5.hpp):

        stepper_base_type::m_algebra.for_each3( m_x_tmp.m_v , in , dxdt_in ,
                typename operations_type::template scale_sum2< value_type , time_type >( 1.0 , dt*b21 ) );

Unfortunately, if value_type's constructor is marked explicit, this will fail. I ran into this trying to use a ceres a jet type for the scalars here for the purpose of automatic differentiation.

It seems like it would be more robust to use something like

const value_type one{1.0};

in these scenarios instead of the raw 1.0. And if I make that change, the ceres jet type works correctly.

I would be willing to work on a PR if this is a change the maintainers would accept.