Finish cleanup in sage/combinat/design

[niles]

The changes in #14499 introduce some failures in optional doctests. The first one should be checked and fixed; the other two come from deprecation warnings.

$sage -t --long --optional=sage,gap_packages sage/combinat/designs/design_catalog.py

**********************************************************************
File "sage/combinat/designs/design_catalog.py", line 8, in sage.combinat.designs.design_catalog
Failed example:
    designs.WittDesign(24) # optional - gap_packages
Expected nothing
Got:
    Incidence structure with 24 points and 759 blocks
**********************************************************************

$ sage -t --long --optional=sage,gap_packages sage/combinat/designs/incidence_structures.py

**********************************************************************
File "sage/combinat/designs/incidence_structures.py", line 528, in sage.combinat.designs.incidence_structures.IncidenceStructure.is_block_design
Failed example:
    BD = WittDesign(9)        # optional - gap_packages (design package)
Expected nothing
Got:
    doctest:1: DeprecationWarning: This method soon will not be available in that way anymore. To use it, you can now call it by typing designs.WittDesign
    See http://trac.sagemath.org/14499 for details.
**********************************************************************
File "sage/combinat/designs/incidence_structures.py", line 626, in sage.combinat.designs.incidence_structures.IncidenceStructure.points_from_gap
Failed example:
    BD.points_from_gap()      # optional - gap_packages (design package)
Expected:
    [1, 2, 3, 4, 5, 6, 7]
Got:
    doctest:1: DeprecationWarning: Unless somebody protests this method will be removed, as nobody seems to know why it is there.
    See http://trac.sagemath.org/14499 for details.
    [1, 2, 3, 4, 5, 6, 7]
**********************************************************************

p.s. Without the deprecation warning, I would have had a tough time figuring out where the first failure was coming from. Thanks for the hint!!

[ncohen]

Sorry 'bout that !

Nathann

[niles]

Applies cleanly, passes all tests, looks good!