Modernize DetDescSvc

• prefer std::unique_ptr over raw pointer and explicit delete
• add const
• prefer STL algorithms over raw loops
• fix ownership transfer
• prefer auto, make constant string static
• remove execute bit from file permissions
• use std::unique_ptr in internal PlacedSolid
• make internal PVolumes keep PVolumeElement by value instead of owned pointer
• use boost::optional is a stand-in for std::unique_ptr when heap allocation should be avoided