FT: Improve DetElement structure and initialization

• remove the empty file, Detector/FT/src/DeFTMat.cpp
• use begin(xx) instead of xxx.begin() format, like using m_mats.begin() instead of begin(m_mats)
• use std::nullopt; instead of std::optional<DeFTMat>{}; etc
• update the copyright of the touch file
• simplify this->access() to access()
• define two functor in Detector/FT/src/DeFT.cpp to simply the code

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Validated by

• Core Software
• RTA
• Simulation