Modernize LumiEvent

• prefer direct member initialization, make constructors = default
• add [[nodiscard]]
• have setters return reference to self.
• move definition of one-line getter/setters into their declarations
• remove addition operators with pointers to objects
• prefer range-based loops
• remove redundant casts
• avoid spurious copies of data
• add some constexpr

to be applied in conjunction with Phys!948 (merged) Phys!938 (merged) and Rec!2444 (merged)