Modernize genconf

• prefer enum over std::string
• prefer freestanding function in anonymous namespace over member functions
• prefer direct member initialization
• prefer auto
• prefer range-based for loops
• use std::type_index to compare std::type_info objects (the latter are not guaranteed to be unique for a given type!)
• prefer std::unordered_map over Gaudi::HashMap
• prefer std::vector<std::pair<std::string,std::string>> over Gaudi::HashMap if no lookup is required