PK_ERROR_code_t        PK_SHELL_ask_type
(
--- received arguments ---
PK_SHELL_t             shell,       --- the shell

--- returned arguments ---
PK_SHELL_type_t *const shell_type   --- the type of the shell
)


This function classifies the given shell according to the types of
topological entities it contains.


The shell type code will be one of:

    PK_SHELL_type_acorn_c            The shell has no edges or faces, but
                                     consists of a single acorn vertex.
    PK_SHELL_type_wireframe_c        The shell has no faces, and one or more
                                     wireframe edges.
    PK_SHELL_type_wireframe_free_c   The shell has no wireframe edges, and one
                                     or more faces.
    PK_SHELL_type_mixed_c            The shell has both faces and wireframe
                                     edges.