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? Answer: if I==L the two vect type are still considered
   different, even when internally they are the same. 

4. So, let's rewrite under these rules:
   psb_mpk_: Always 32 bits, used for MPI related stuff.
   psb_ipk_: Can be 32 or 64 bits, always used for "local" indices and
   	     sizes
   psb_lpk_: Can be 32 or 64 bits, always used for "global" indices
   	     and sizes, must be psb_lpk_ >= psb_ipk_
   psb_8pk_: always 64 bits, used for SIZEOF & friends.