Adopt hierarchical principle

[ppKrauss]

We can interpret the Frictionlessdata general principles of simplicity, extensibility and reuse as rationale to the "hierarchical principle"... Or, in a most direct analogy with CSS (or JSON-LD, Microdata, etc.) reuse the parent as default value.

We can evoke the hierarchical principle for frictionlessdata/datapackage#42 (lang use) and frictionlessdata/datapackage#343 (rdfType use).

When we need the descriptor as scope for an inner-descriptor, we can use the value of the parent as scope or as default value... At this example we used lang and rdfType at "source level", as default value for the "field level".