[Futurev4] Use DD4HEP in a dedicated platform

See merge request !799

[Futurev4] Extract detector related conf to Gaussino & add tests for the external geometry

See merge request !778

[Futurev4] Add a test for running Gauss with DD4hep geometry and fix problems

See merge request !725

[Futurev4] Make FindPhotos compatible with master

See merge request !718

[Futurev4] Move to LCG layer LHCB_7

See merge request !796

[Futurev4] Remove std::array from the Gaudi::Functional constructors

See merge request !769

[Futurev4] Updating how LCG layers are introduced

See merge request !719

[Futurev4] fix failing tests

See merge request !686

The test is based on the example at
https://gitlab.cern.ch/Gaussino/futuregaussexamples/-/blob/854f8978601a669342101901aa74927a2730c6fb/dd4hep/main.md

The test is placed in a temporary new package (Sim/FutureTests) since
the tests in Sim/Gauss are disabled.

Until this change, the writer GaussTape was run too early by the
AvalancheScheduler because its dependencies are not declared.

Here the GaussTape writer and the packers are now wrapped into
GaudiSequencer-s which tells the AvalancheScheduler that GaussTape
must run after the packing. Similarly, MCTruthToEDM is explicitly
scheduled before the packers (which also don't delcare their inputs).
With these changes, one can write out a .sim file as long as a single
thread is used.

EvtGenDecay.ImHelOne was wrongly named RealHelOne causing a collision.
As a result the property `m_imHelOne` had a value of 1 instead of 0.

[Futurev4] Make G-on-G py3 compatible

See merge request !753