Implementing PPMPF_TUP_ATPOS, PPMPF_TUP_SPLIT using PPMPF_COMPOSE.

Several changes have been made that take advantage of preprocessor function
macro composition using PPMPF_COMPOSE.

PPMPF_DPAR is going to substitute PPMPF_DREF gradually while the latter goes
gradually into internal use mode.

Given a ppmpf tuple with N items and K an index ppmpf number such that K < N,
we can get two ppmpf tuples representing the ranges [0,K) and [K,N) using the
PPMPF_TUP_SPLIT function macro. PPMPF_TUP_SPLIT is then implemented using
function macro composition through PPMPF_COMPOSE. Then composition is used
again to rewrite the implementation of the PPMPF_TUP_ATPOS function macro.

All the ppmpf tuple functionality is now accessible through a single header
due to the ongoing refactoring of ppmpf itself (i.e. <odreex/ppmpf/tuple.hh>).