1 - merge_conditions now deletes corresponding condition.  Added a
condition pointer to condition struct to link variablized condition to
instantiated condition so that we can properly delete instantiated
conditions if the corresponding variablized condition is deleted when
merging conditions.
2 - Added function to clean up chunk_cond list.  Deletes chunk_conds
whose variablized condition has been deleted.  Bit tricky b/c 
variablized conditions may have NC and NCCs that do not have 
instantiated equivalents.
3 - Re-wrote reorder_instantiated_conditions.  Now that we have a link
between variablized conditions and instantiated ones, we can re-order in
a more straightforward way.
4 - When building chunk conditions for grounds, we no longer copy
relational tests.  Not sure if it was causing a problem, but there were
some duplicate tests being caused by it.  Flagged to check later.