Fix warnings - Githubissues

fix a few warnings when compiling with gcc and clang

This misses a few warnings like this from gcc (not clang):

/home/thomasn/dev/audio/libadm/src/private/rapidxml_formatter.cpp:36:24: warning: ‘*((void*)(& cartesianPosition)+20).adm::detail::NamedType<float, adm::ZOffsetTag>::value_’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   36 |           node.setValue(element.get());
      |           ~~~~~~~~~~~~~^~~~~~~~~~~~~~~
/home/thomasn/dev/audio/libadm/src/private/rapidxml_formatter.cpp:565:14: note: ‘*((void*)(& cartesianPosition)+20).adm::detail::NamedType<float, adm::ZOffsetTag>::value_’ was declared here
  565 |         auto cartesianPosition =
      |              ^~~~~~~~~~~~~~~~~

I can't quite figure out the cause of this. boost::optional<T>::get just asserts that there is a value (doesn't throw an exception), so PositionOffset::get<ZOffset> can return an uninitialised value, but this code doesn't return unless has returns true, so that never happens.

I suspect the compiler just gives up trying to work out what's happening in this specific case (lots of templates). Throwing an exception in OptionalParameter::get if the optional is empty seems to solve this, and would probably be a good idea, but it's a fairly big change to apply this to the non-auto-base types too.

Impacted Files	Coverage Δ
include/adm/detail/id_map.hpp	`100.00% <ø> (ø)`
include/adm/private/rapidxml_utils.hpp	`100.00% <100.00%> (ø)`

ebu / libadm

Fix warnings #161

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