Implemented use of "boost::optional" for some of the instances where "boost::none_t" was already being used:
mapping, unit, repository.
Not implemented for many yet to clarify cases: "dataType", "labels", "extents", "units", "values", "link", "extent", "metadata".
Merge pull request #128 from balint42/optional_branch

Implemented use of "boost::optional" for some more cases

Part of issue #86