Nils Erik Krumnack authored 1 week ago and Frank Winklmeier committed 1 week ago

support for std::vector<T> columns in columnar tools

This is fairly straightforward in xAOD mode, but in columnar mode it is
somewhat involved, as it needs both a data column and an offset column.
Beyond that, the handling of type conversions requires a custom range
adapter.  This MR doesn't quite demonstrate the need for type
conversions, but they will be needed for std::vector<ElementLink<T>>, in
a subsequent MR.

One thing I'm not 100% sure about is whether the type ought to be
std::vector<T> or use a custom type for this purpose.  In the prototype
I had used columnar::ColumnVector<T>, but that may create more confusion
than it removes.  The main concern about using std::vector<T> is that
the user doesn't receive an std::vector<T> but an std::span<T> or other
range type.  Still, in many regards they behave like a vector and
reusing the STL type may be clearer.