Revive output_parallel_statistics and make it a plugin

[tjhei]

see

$ grep output_para -i -r  source/
source/global.cc:  const bool output_parallel_statistics = false;
source/simulator/helper_functions.cc:    if (!aspect::output_parallel_statistics)

Turn this into a postprocessor that can be turned on instead of a variable that you have to modify by hand. Make sure it works, maybe extend it.

[gassmoeller]

@spco is this issue closed by #1062 or is there something else coming?
Thanks for working on this by the way, I will definitely use this postprocessor for some of my runs 👍

[spco]

This can be closed - the matrix statistics I'm working on will go towards #1053.