Fix styling of families included in node groups.

Closes #363;
Removed use of non-existent graphviz style 'unfilled'.
Added a comment on visualization with MI.
Added graphviz reference URL to the user guide.
Allow "inherit=None,parent" to make parent secondary.

Addresses #369
Added some verbose validation output.
Merge pull request #370 from hjoliver/369-demote-first-parents

369 demote primary parents with inherit=None,parent
Merge pull request #366 from hjoliver/363-vis-families-in-node-groups

Fix styling of families included in node groups.