Correct volume averaging index in correlator computation for online measurements.
Significantly reduces variance of measurement.

In source_generation_* for pion and nucleon sources, use seed compatible
with a serial run. For all source generators which reset the seed,
increase luxury level to 2 by default.
Merge pull request #259 from kostrzewa/source_generation_seed

fixes related to online measurements and source generation
Merge remote-tracking branch 'etmc/master' into smearing_openmp