A fast random access macro function for ppmpf tuples.

Should one have a need to randomly access individual elements in ppmpf tuples,
a dedicated macro construct allowing such access with a minimum amount of
macros involved is necessary. PPMPF_TUP_ATPOS serves exactly that purpose with
index values ranging from (0)(0)(0)(0) to (9)(9)(9)(9), as numbers are used
within ppmpf itseld.