Documentation: enhanced doc for constructors

- Better explanation of the default constructor
- Better explanation of the copy constructor
Documentation: EntityWithSources::addSource

- Improve comment as suggested by jan
Documentation: s/limited by depth/limited in depth/
Documentation: better doc for methods with filter

According to jans comment the following replacements were made:
- s/accepts every/accepts all/
- s/containing the filtered/containing the matching/
Documentation: better doc for DataArray
Documentation: s/si unit/SI unit/
Documentation: better doc for DataArray
Documentation: improved doc for Dimension
Documentation: improved doc for DataTag
Documentation: improved doc for Feature
Documentation: improved doc for SimpleTag
Documentation: small fix in ImplContainer