Add LOGGER functor

Gerhard Ravenrequested to merge
add-logger-functor into master

LOGGER: a functor which print a message, potentially containing the value of its argument and its type (use {arg} and {arg_type} as part of the message to do so), and then returns its (single!) input argument.

Motivated by an attempt to debug #596

Edited by Gerhard Raven

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