Especially useful when all nonlinear iterations should
be output. See <output_iteration_results> tag.

The fmt library happily accepts std::string without
any conversions.

Including
 - usage of static_cast instead of c-style cast,
 - single variable declaration per line,
 - removing const from return types
 - omit unnecessary string to char* conversions,

The used clang-tidy checks are:
google-readability-casting,
readability-const-return-type,
readability-container-size-empty,
readability-delete-null-pointer,
readability-deleted-default,
readability-else-after-return,
readability-function-size,
readability-identifier-naming,
readability-inconsistent-declaration-parameter-name,
readability-isolate-declaration,
readability-misleading-indentation,
readability-misplaced-array-index,
readability-named-parameter,
readability-redundant-*,
readability-simplify-*,
readability-static-accessed-through-instance,
readability-static-definition-in-anonymous-namespace,
readability-string-compare,
readability-uniqueptr-delete-release

The move ctor is not implemented in gcc < 5.

Clang format run on near-the-header lines.

Happy New Year

no special behaviour if second path starts with dot
the old behaviour could cause problems with unix' hidden files or with parent
directories (..)

It is defined multiple times through different translation
units.

This removes the clang sanitizer error "runtime error: unsigned integer overflow: 0 - 1 cannot be represented in type 'size_type' (aka 'unsigned long')" in the test