Introduced information flow whether the timestep was accepted or not.

This prevents the times in the timestepper and the external loop to diverge
which causes two issues otherwise:
1.) Endless loop at t --> t_end, if max_iter > largest multiplier
2.) Endless loop at t --> t_end, if nonlinear solver fails during
	execution

If the number of iterations is smaller than the first
entry in the iterations vector, the multiplier will be
set to the first entry value.

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.

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.

Clang format run on near-the-header lines.

Happy New Year

 a new member.

This potentially saves a second copy compared to using const
reference. See Effective Modern C++, Item 41.