trac #16553: Clean IncidenceStructure
trac #16553: is_t_design
trac #16553: Review
trac #16553: doc fix + deprecation is_block_design
trac #16553v3: change .points() -> .ground_set()
trac #16553: merge sage 6.3.beta5
trac #16553: deprecated alias .points() + fix
Trac #16553: Clean IncidenceStructure

In #16552 we discovered that `IncidenceStructure` is not adapted to deal
with projective spaces. The class should be able:
 - to be a base class for inheritance to create other designs (like
`GDD` or specialized one such as `DesarguesianProjectivePlane`)
 - ... (maybe more)

see also: #16534

URL: http://trac.sagemath.org/16553
Reported by: vdelecroix
Ticket author(s): Nathann Cohen, Vincent Delecroix
Reviewer(s): Nathann Cohen, Vincent Delecroix