improve time information in TimeLoop

Feature request

There are two small issues about the output of time information in the class TimeLoop.

1. The output format of wall clock time and other CPU times is in a low precision {:.2g}, so it is impossible to measure the time in detail.

2. (Optional) The output shows Simulation took but Time loop took seams more consistent since only the time of the time loop is measured.

For parallel computations the start-up (partitioning etc.) may take a lot of time and this time belongs also to as simulation, but is not measured by TimeLoop.

What does this feature / why does DuMux need it:

• Change the precision to {:.5g} for Wall clock time, Simulation took and the cumulative CPU time

• (Optional) Rename Simulation took to Time loop took

DuMux needs a higher output precision for time related information to allow users to measure the time used by the time loop for performance studies, benchmarks and checking/plotting the convergence. Making a plot of **Wall clock time** and time or time step size only works if all variables are given in a sufficient precision.

Which issue does this feature fix (if any)

Anything else we need to know?:

This issue is related to #916 (closed) I'm not sure if the formulation Simulation took has been explicitly chosen instead of something like Time loop took.