- renamed readableName to title (to be consistent with root naming conventions)
 - made nBins in AxisSpec std::optional to seamlessly distinguish between fixed and variable binning per axis -> removed binsEqual in AxisSpec and HistogramConfigSpec
 - added callSumw2 property to HistogramSpec
 - rename axis lable -> title (to be consistent with root naming conventions)
 - added optional name property to axis (this can be helpful in n dimensional histograms so they are not only called axis0, axis1, axis2...)
 - added possibility to insert axes also after ctor was called
 - store histograms in registry in form of a std::variant (HistPtr) that can take any root hist pointer type
 - added static HistFactory helper class that handles the creation of the actual root histograms
 - HistFactory provides createHist<>() function to instanciate arbitrary histogram based on the blueprint defined in HistogramSpec
 - HistFactory provides createHistVariant<>() function that creates the histogram and returns a properly casted HistPtr which can be stored in the registry
 - HistFactory provides a runtime version of the above, which creates the histogram based on the type information stored in HistogramConfigSpec
 - it was necessary to move from unique_ptrs to shared_ptrs because we must be able to 'downcast' the actual pointer to the few interface types stored in HistPtr (this can not be done with unique ptrs). As a beneficial side effect, this way the pointers created by the HistFactory can also be stored in OutputObj<> (which uses shared_ptrs)
 - extend HistogramCreationCallback to cover all root histogram types
 - moved HistogramCallbacks map to .cxx as it is now const static member variable in HistFactory that should be defined only once (otherwise linker will complain)
 - get() now also requires to specify the interface type (TH1, TH2, TH3, THn, THnSparse, TProfile, TProfile2D or TProfile3D) since type cannot be infered from the variant at compile time
 - allowed adding histograms to an existing registry
 - added HistFiller static helper class that contains functions to fill histogams of any type both simply via values or via filtered tables
 - extended existing fill-from-table functionality to arbitrary number of dimensions
 - added fill and fillWeight functions to registry that can fill the stored histograms (again providing a version to fill values and one to fill whole filtered table)
 - renamed HistogramType -> HistType to reduce code verbosity