The unpdl method is roughly an inverse of the pdl constructor.
It takes a piddle and returns a reference to a Perl array containing
a dimensionally equivalent structure of nested arrays. This complements
the list method, which returns a flattened list
of Perl scalars.

This will allow development versions of PDL to
show the symptom with the hopes of providing
enough info to debug the problem for the developer.
Also, replaced print by diag in test output.