Update to C++17 standard

[svengoldberg]

Description

This PR updates the C++ standard to 17 to make use of newer coding features.
Moreover, the standard was also defined in the src/CMakeLists.txt which seems redundant. Now, the C++ standard only needs to be defined in the <ROOT>/CMakeLists.txt.

Task	Finished	Reviewer Approved
At least one test for the new functionality was added.	yes does not apply	OK
New classes have been added to the Python interface.	yes does not apply	OK
The code is properly documented with doxygen docstrings	yes does not apply	OK
Changes are documented at the top of ChangeLog.md	yes does not apply	OK

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.47%. Comparing base (c5ccbb2) to head (0d8b557).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1045   +/-   ##
=======================================
  Coverage   69.47%   69.47%           
=======================================
  Files         301      301           
  Lines       26889    26887    -2     
=======================================
  Hits        18681    18681           
+ Misses       8208     8206    -2     

Flag	Coverage Δ
unittests	69.47% <ø> (+<0.01%)	⬆️

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see 1 file with indirect coverage changes