Changing include order causes an internal Eigen error:
Eigen/src/IterativeSolvers/InternalHeaderCheck.h:2:2: error:
   #error "Please include unsupported/Eigen/IterativeSolvers
   instead of including headers inside the src directory directly."

If possible the scaling include should be removed in the future.

The Sparse' module parts are used also if unsupported
module support is turned off.

add gmres restart test1

set gmres restart test2

add restart option for all Preconditioners (final)

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

Include spdlog via BaseLib/Logging.h, which
provides macro replacements for
ERR/DBUG/WARN/INFO...

Fixes multiple copyright messages in the doxygen docu.

ag '\\file' -L --ignore=ThirdParty *Lib |xargs  sed -i 's/\(.*\)\\copyright/\1\\file\n\1\\copyright/'

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

Clang format run on near-the-header lines.

Happy New Year