PPSD: enable getting/plotting mode and mean psd values
Merge branch 'master' into ppsd_stats
minor refactoring
add PPSD tests

 - basic tests to pin get_mode(), get_mean() behavior
 - also a PPSD.plot() image test included now
Update CHANGELOG.txt
Merge pull request #804 from megies/ppsd_stats

PPSD: enable getting/plotting mode and mean psd values