Placeholder: allow representation as lp.ValueArg (#587)

* Placeholder: allow representation as lp.ValueArg

* mypy

* Update doc

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* minor fixes

* fix doc

* add simple test

* Tweak test to ensure ValueArgs are used

* Don't use assert for error checking

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Co-authored-by: Andreas Klöckner <inform@tiker.net>