Fix usage of stdcxx::optional with C++-17 compliant compilers

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[x] The commit message follows our guidelines
[ ] Tests for the changes have been added (for bug fixes / features)
[ ] Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem ? If so, link to this issue using '#XXX' and skip the rest No

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) Bug fix (fix compilation with C++-17 compliant compilers)

What is the current behavior? (You can also link to an open issue here) With C++-17 and newer versions, stdcxx::optional = std::optional. With older compilers, stdcxx::optional = boost::optional. Sometimes, we called stdcxx::optional::get() which is boost-specific => the library does not compile with recents compilers.

What is the new behavior (if this is a feature change)? Compilation is OK with C++-17 and newer compilers

Does this PR introduce a breaking change or deprecate an API? If yes, check the following:

[ ] The Breaking Change or Deprecated label has been added
[ ] The migration guide has been updated in the github wiki (What changes might users need to make in their application due to this PR?)

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powsybl / powsybl-iidm4cpp

Fix usage of stdcxx::optional with C++-17 compliant compilers #412