Add variadic to add_axis and add_mark

[gouarin]

With this improvement, we can do

fig.add_axis(ax_x, ax_y, ax_z);

At the moment, we can only add one axis or one mark at a time.

[SylvainCorlay]

I like the new api to insert with initializer lists

vec.insert(vec.end(), {
    ax_x,
    ax_y,
    ax.z
});

I wonder if we should have a type of observed sequence with STL-style API.