#311: Don't support Node Expressions as constraint parameters in Core (#364)

Closes #311

* #311: Don't support Node Expressions as constraint parameters in Core

* #311: Introduced term parameter value to allow NEs in other documents, use it where allowed

* Update shacl12-core/index.html

Co-authored-by: Andy Seaborne <andy@apache.org>

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Co-authored-by: Andy Seaborne <andy@apache.org>

Author: HolgerKnublauch