Members of physics::Constants must be instantiated, since they might be odr-used

Some constants are already instantiated, but other aren't. Some compiler complain about it (if C++17 features are not supported), and give link errors of the form

```
/usr/bin/ld: CMakeFiles/potential_temperature.dir/potential_temperature_test.cpp.o: in function `void scream::run, Kokkos::Device >(std::mersenne_twister_engine&)':
/pscratch/sd/n/ndk/wacmy/s12-mar1/components/scream/src/diagnostics/tests/potential_temperature_test.cpp:174: undefined reference to `scream::physics::Constants::P0'
```

The fix is super easy: at the bottom of the file, for all constants do what's already done for some of them.