Added macros for ppmpf hexadecimal recursion depth maximum.

 *) PPMPF_IEXEX: converts internal hex representation to 0xNNNN
 *) PPMPF_RDMAX: if not set, defaults to 65535 -> (15)(15)(15)(15) -> 0xFFFF
 *) PPMPF_RDMH4(), PMPF_RDMH3(), PPMPF_RMDH2(), PPMPF_RDMH1() are the internal
    "digit" representations for hexacedimal recursion depth numbers.

NB: PPMPF_RDMAX() is derived from PPMPF_RDMAX as defined during compilation to be
    a decimal number (0 - 65535).
Making PPMPF_FLDX0G dependent of PPMPF_RDMAX predefined depth.
Copyright year fix in <odreex/ppmpf/algorithm/fldaux.hh>

Much of the code introduced in this header was excised from its fold.hh
counterpart during refactoring; fldaux.hh in this case is thus derived
from the 2013 codebase in odreex/ppmpf and therefore this change is
justified.
Towards taking care of safe / unsafe ppmpf tuples.

A rather complicated commit, aiming towards deprecating the older recursive
expansion macro construct. Safe and unsafe ppmpf tuples have separate fold,
map, zip, zipwith high order macros. The addition of recursively expanding
"double" folding construct targets self - reentrant issues when multiple
"higher order" macro functions are combined with ppmpf tuples.
Cleaning up previous fold implementations in ppmpf.
Several changes to ppmpf sequences, resolves #1

The new recursive expansion construct is used; folding, mapping and to tuple
transformation fixed accordingly.
Added missing tests for ppmpf sequence folds.
Added missing test for ppmpf sequence constructor.
Merging recursive expansion consruct refactoring, ppmpf collections' change.