Dune::Timer includes two alternative implementations
for measuring time. Historically one was based on getrusage()
and the other one on std::clock(). The former was the default
and the latter could be enabled by defining a macro. Both in fact
measured the time spend computing by the process.
In fa43f4bf the default code path was changed from
getrusage() to std::chrono::high_resolution_clock::now()
while std::clock() could still be enabled by the macro.
This was in fact a breaking change because the new default
version measures the elapsed real time.
This patch removed the non-default version based on std::clock because:
The breaking change happened almost 11 years so one can consider the
new behavior the established 'correct' version.
The macro switch was neither documented nor tested.
Setting the macro manually in user code is error prone.
Both versions do completely different things leading to
different measurements. It's not even clear that one
produces larger numbers in general:
std::clock adds up time spend computing in all threads of the
process. Thus time in concurrent threads is added up, while
the time a thread is sleeping is not counted.
There a minor grain of salt: The documentation still documented
the old behaviour and instead of 'fixing' the code this patch
adjusts the documentation to established reality.
