clang-tidy modernize-use-transparent-functors check.

clang-tidy modernize-use-override check.

clang-tidy modernize-use-default-member-init check

Also those where the second and so forth arguments are defaulted.

The MathLib::TemplateVector3 is an exception, because of the many
required changes through the code, which relies on the implicit
conversion from TemplatePoint<T, 3> to TemplateVector3<T>.

clang-tidy '*explicit*' check.

Change the way a multiplier is computed, s.t. for
a set of iterations and multipliers as:
i = 5 8 10
m = 2 1 0.5
following multiplier is taken
m(<5) = 2
m(5-7) = 2
m(8-9) = 1
m(>=10) = 0.5

This is easier to comprehend in the project files.

Otherwise the timestepper will repeat the second iteration
of the first time step with the initial_dt and simulation will
be aborted. In the current implementation the minimum dt
will be taken for the second iteration.

Check vector sizes.

Remove duplicate docu from the inherited functions.

The iteration number is now set through the next() call.

It contains a boolean for met error norms and a
number of iterations of the solver made.

The number of iterations is needed for
a timestepping scheme.

It is obviously adaptive.

Obligatory clang-format too.

Choosing int over size_t for number of iterations -- nobody
is going to wait that long anyway.

std::vector<std::size_t> _ids_of_active_elements.

 improvement in SerialExecutor

If the derived boundary mesh dof table is defined only on base nodes
then the other nodes do not contain a valid global index.