Move old unit tests for Source
Add assignment operator for nullptr_t to Source

Add missing operator<< to Source
Method testOperators was not registered on TestBlock
Started unit test for nix::Source
Add missing implementation for Source::sources()

Harmonize some implementations of other methods of SourceHDF5.
Remove TODOS and commetnted code parts.
Unit test for nix::Source done
Implement method findSource
Some changes according to the comment of @gicmo
Merge pull request #47 from stoewer/master

Unit test for nix::Source

Fixes the Source part of the unit tests meta-issue #25.
Reviewed and tested locally.