Refactored UnplacedVolume
This is adiabatic step #1 in our current restructuring process to VecCore + new class layout.
This merge request contains:
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introduction of global vector and scalar backends from VecCore to be used in implementations
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proposition of the new UnplacedVolume interfaces
UnplacedVolume might still be uncomplete w.r.t. to USolids. We need to discuss what more to include.
Despite new interfaces, I have made sure that the current code still compiles fine. However, this means that the interfaces cannot be pure virtual for the moment.