Fix LGSSM parallel filter likelihood

[nkbranigan]

• Fixes Marginal log likelihood of LGSSM parallel filter does not match recursive filter #423
• The issue found that LGSSM’s serial and parallel filters gave different likelihoods when the initial mean was large
• The problem was that the first message’s likelihood was missing a term with the initial mean
• Added the repro from the issue as a test, verifying that after this fix the serial and parallel filters agree