Internally Use optional-lite

[ax3l]

@franzpoeschel would like to use a preview of std::optional internally (without crossing the public API).

We could use optional-lite this in C++11.

Originally mentioned in
#482 (comment)

[franzpoeschel]

Alternatively, we already have a backport of std::variant in the openPMD API which can be used to quickly implement an optional type.
Since I needed this on my streaming branch, I did just that in a recent commit.

[ax3l]

Sounds good, feel free to expose it as a separate header object in a PR already.