Rework Expressionparser

Description

If the expression is constant after the replacing steps, then the result can be added to all target rows and not just to the minimum of all variables. (Check out unittests to see examples) For example mean(x) returns a single value independet of the size of x. Therefore it can be used in all target rows

• Iterate over complete yVector, if the expression is indipendet of the source data in xVector
• Moved a lot into Expressionparser, because so it is easier to determine if the Expression is constant.
• Better error handling in the parser
  • Check the number of arguments of the functions
• Speedup parsing and therefore calculating values.

Conformity

• Changelog entry
• Unit tests
• Update INSTALL
• Downport
• Fixes: https://bugs.kde.org/show_bug.cgi?id=474732

When external dependencies are removed

• Reporting to ...