Cleanup initialisation of dimensions

[tpadioleau]

The current way of initialising dimensions is verbose:

auto const [x_domain, ghosted_x_domain, x_pre_ghost, x_post_ghost]
        = ddc::init_discrete_space<DDimX>(DDimX::init_ghosted<DDimX>(
                ddc::Coordinate<X>(x_start),
                ddc::Coordinate<X>(x_end),
                ddc::DiscreteVector<DDimX>(nb_x_points),
                gwx));

The name of the dimension can appear multiple times for no good reason.

I suggest to have a simpler API with only create_* functions that will take care of calling ddc::init_discrete_space. We would have

• create_uniform_point_sampling<DDim>
• create_uniform_point_sampling_ghosted<DDim>
• create_non_uniform_point_sampling<DDim>
• create_non_uniform_point_sampling_ghosted<DDim>
• create_uniform_bsplines<DDim>
• create_non_uniform_bsplines<DDim>

The function ddc::init_discrete_space would remain a public function but would not be called directly by the user when initialising a dimension.

[tpadioleau]

see branch archive/explore-alternative-initialization