interface convergence between balls and intervals

[vdelecroix]

The real balls (e.g. RBF) and the real intervals (e.g. RIF) are meant to achieve the same goal: provable real computations. This task ticket stands to discuss how to unify their interface (and possibly adapt the other floating point fields as well).

• clean the clumsy c++ dependency #24285
• should the parent be extension classes? In particular, is it wiser to have function factories or inherit from UniqueRepresentation?
• Creation of new elements in Cython. This boil down to some allocation together with affectation of the parent. Should this be done in __cinit__ (as it is the case for RIF) or __init__? We might definitely want to let the possibility of having pools (see #17670).
• semantics of comparisons