Fix/compiler warnings 834

[barendgehrels]

This fixes #834 and two additional new compiler warnings

[vissarion]

Looks OK, but tests are failing on clang-9.

[barendgehrels]

Looks OK, but tests are failing on clang-9.

Must be a false negative

 In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/mutex:39:
/usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/chrono:1345:24: error: unknown type name 'strong_ordering'
      friend constexpr strong_ordering

[barendgehrels]

I'm running them again

[awulkiew]

Yes, it seems that clang-9 problem is not in Geometry. I think it may be ignored.