Fix fluctuating LB with zero centered pdf storage

Based on Frederik's work. I replaced an additional zeroth_moment_symbol by density_symbol and adapted the test such that the tolearnes on all fluctuating quantities scale with square root of system size.

assigned to @da15siwa

requested review from @holzer

added 1 commit

• eb4a8f98 - Use Guo forcing

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mentioned in commit c7d65a7d