Piraha lookup ref issue

[Steven R. Brandt]

Flesh: Currently, lookup_var returns a reference to the value of a var. When a unop is called, this has the effect of negating the value in the table. Thus:

    $zip = 1
    foo::xmin = -$zip
    foo::xmax =  $zip

This results in both xmin and xmax being set to the same value (-1). The fix is to have lookup_var return a copy rather than a reference.

[Steven R. Brandt]

PR is here: https://bitbucket.org/cactuscode/cactus/pull-requests/165