[Feature request] Allow for default class where a rdf:Class does not exist.

[charlesivie]

Is your feature request related to a problem? Please describe.
When data does not have a class type
e.g. nothing like
:Charlie a :Person
The user interface cannot display a node, as it is reliant on that data to be in the model.

Describe the solution you'd like
Where a class does not exist for a node, assume a default class type of rdf:Resource.
Update the SPARQL queries accordingly using 'Coalesce'.

[xiazcy]

Relevant issue: #185.