Resolve "thrill GA talk"

Closes #111

See merge request !49

Resolve "dot preview: text is not centered inside node"

See merge request !51

Resolve "Reference node: display referred node name in diagram"

Closes #104

See merge request !48

Resolve "Properties: set_value function: Check value type."

Closes #98

See merge request !46

Add corresponding unit test.

Add powerpoint talk for THRILL workshop.

Resolve "Add documentation for property system"

Closes #60

See merge request !45

Add corresponding unit test.

Resolve "Group node: set inverted -> add node"

Closes #39

See merge request !44

Add corresponding unit test.

Resolve "Improve property system"

Closes #46

See merge request !42

Resolve "Display errors in examples"

Closes #90

See merge request !43

Resolve "graph of deserialized group nodes does not contain the nodes of group"

Closes #89

See merge request !41

Update unit tests accordingly.  Groups seem to work now.

still producing port mapping bugs...

Resolve "Group: Add analysis unit test"

Closes #80

See merge request !40