Rename gradientAtQp into gradientOf and similarly other expressions

Praetorius, Simonrequested to merge
issue/cleanup_expressions into master

Summary

This renames some expressions:

  • gradientAtQP -> gradientOf
  • discreteFunction -> valueOf
  • derivativeAtQP -> derivativeOf
  • partialAtQP -> partialDerivativeOf
Edited by Praetorius, Simon

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