Merge pull request #4993 from RosettaCommons/vmullig/fix_unit_test_werror_clang12
Fix a warning-as-error in the unit test suite, raised by clang 12
Fixing the following error raised by clang 12's warnings-as-errors:
```
./test/protocols/indexed_structure_store/utility.hh:96:21: error: loop variable 'pair' is always a copy because the range of type 'combined_range<decltype(boost::make_tuple(boost::begin(rngs), boost::begin(rngs)))>' (aka
'combined_range<tuple<ndarray::detail::StridedIterator<protocols::indexed_structure_store::ResidueEntry>, ndarray::detail::StridedIterator<protocols::indexed_structure_store::ResidueEntry>, boost::tuples::null_type,
boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type> >') does not return a reference
[-Werror,-Wrange-loop-analysis]
for ( const auto & pair : boost::combine(a, b) ) {
```