In particular, include standard library headers as needed, fix
function declarations (operator<<() for Well::WellProductionProperties),
and make a few helper functions 'static' to avoid warnings of the
form "no previous declaration for".
For the Schedule's keyword handlers, also switch to storing member
function pointers directly instead of std::function objects. This
saves space and does not incur function pointer conversions. Use
std::invoke to call those handlers to avoid having to spell out the
'->*' operator.
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