1. Perhaps we should switch completely to external indices being of
   kind LPK and internal indices being of type IPK, and the two morph
   independently in int32/int64 (although admittedly LPK=32 and IPK=64
   does not  make much sense)
2. Change sizeof_ constant names accordingly to sizeof_ipk and
   sizeof_lpk.

3. Should we define a psb_l_vect_type? But then, if I==L how can we
   distinguish?