value_type cannot have an explicit contructor

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](https://github.com/ceres-solver/ceres-solver/blob/master/include/ceres/jet.h#L223) 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.

Start with the example in runge_kutta_dopri5.hpp and search odeint for similar scale_sum constructions using raw 1.0 values. Check how these expressions behave with an explicit value_type constructor, then verify that the affected code supports the ceres Jet type without breaking existing scalar types.

Written by the indexing model from the issue text.