ISSUE-49 Meaning of Filter Shapes

The spec is currently not clear about the meaning of filter shapes.
They are tied in with the scoping mechanism. However, they are useful
independently of scoping, e.g. applied to property constraints.

A natural interpretation of filters is as an IF-THEN construct.
* If the filter is not satisfied then the shape is satisfied, i.e. no
violations can be raised by the constraints in the shape.
* If the filter is satisfied then the shape is satisfied if and only
if all the constraints in the shape are satisfied.

With this definition, scopes and filters are independent features of
SHACL. filters are always used when determining if a shape is
satisfied.

-- Arthur