- solveAlgebraic: adjusted to change of changeGrid from call-by-reference to call-by-value
- diff(): now supports specification of various variables for which the derivative should be applied. Ensured consistency with Symbolic. Maybe a more elegant use of gradient() is possible.
- unittests: adjusted for now behavior of diff()

quantity.Symbolic bugfixes:
- diff(): now supports specification of various variables for which the derivative should be applied. Ensured consistency with Discrete.
- unittests: adjusted for now behavior of diff()