Update to Gaudi v38

Update athena projects to Gaudi v38r0.

The main braking change is that in Gaudi v38, std::unordered_set properties are now represented as python set instead of list. This mostly affects CondInputLoader.Load and the Algorithm's ExtraInputs/Outputs property. For details see gaudi/Gaudi!1503 (merged) but in practice it means that code like

   MyAlg.ExtraInputs += ['foo']

has been updated to

  MyAlg.ExtraInputs.add('foo')

For the moment only std::unordered_set is mapped to a Python set. Regular std::set remains as Python list to maintain insertion order.

For backwards compatibility, we allow implicit conversion from list, i.e. the following still works:

   MyAlg.ExtraInputs = ['foo']  # implicitly converted to {'foo'}

In addition some minor const interface changes in DataHandle that propagate to our VarHandleKey.

cc @leggett