Before WP<->P implicit conversion changes, one could choose to not keep hidden
points by using `Triangulation_cell_base_3` as cell base instead of
`Regular_triangulation_cell_base_3`. This is not possible anymore as point types
will conflict.
This changes introduces a new template parameter to pass a policy on whether
to keep or discard hidden points.
It is a breaking change since it is placed before the container template
parameter, but it makes more sense that way and the container type template
was not documented.
