only open log.txt on processor 0

Only processor 0 will be writing into the file output/log.txt anyway, so
we only need to open it there. This might also fix some filesystem race
conditions.
Merge pull request #211 from tjhei/fix_log_txt_creation

merged
Add the option to increase output resolution.
Merge pull request #213 from gassmoeller/finer_output_mesh

Add the option to increase output resolution.