Implement support for `sh:or`

[bartkl]

Rudimentary support for sh:or suffices for now. The form we're looking to support with priority is:

    sh:property [
          sh:path vocab:fromEtoC ;
          sh:or (
            [ sh:node shape:AShape ; ]
            [ sh:node shape:CShape ; ]
          ) ;
          sh:minCount 0 ;
          sh:maxCount 1 ;
      ] .

I've started the work for this on the feature/sh-or branch.

I'm currently waiting for a Lancaster issue to be solved which should pretty much complete this task.

Make sure to extend the documentation in the wiki accordingly.