When computing the variations in a background thread, the
code has to work on a copy of the dive plan and the deco
state. Instead of passing a copy via a unique_ptr<>, simply
use value semantics when calling computeVariations().
This does an unnecessary copy of the deco state, when
computeVariations is not run in the background, but so what.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
