Fixes for gcc 13, clang 16 and C++20

• Fix implicit capture of this (deprecated in C++20)
• Fix (deprecated) implicit conversion from enum to float
• Avoid use of deprecated is_pod
• Replace unneeded enum with constexpr unsigned int
• Remove unused variables
• Fix implicit copy constructor/assignment deprecation warning
• Use && instead of & for booleans
• Hide null-pointer-subtraction warning
• Fix potential dereferencing of nullptr
• Hide warnings 'void operator delete(void*, std::size_t)' called on pointer '<unknown>' with nonzero offset from BernsteinPoly.cpp (probable false positive)
• Fix warning about unused functions (only in standalone build of public headers)
• Replace std::random_shuffle with std::shuffle (random_shuffle was deprecated in C++14 and should have been removed in C++17) moved to !4335 (merged)

Notes:

• when building *-gcc13-dbg platforms there are a number of spurious warnings in dictionaries, fixed by gaudi/Gaudi!1501 (merged)
• commit d8ad2fe3 (from random_shuffle to shuffle) requires a change in the reference files in Boole and Rec