Add Laplace approximation as algorithm#16

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bbbales2 wants to merge 8 commits intostan-dev:masterfrom

bbbales2:feature/hessians_optimize_cmdstan

Commits on Mar 8, 2020

Added proposal for adding Hessian of log density in optimizing output for cmdstan
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Fixed some sign stuff, specified where normal approximation comes from
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Commits on Mar 15, 2020

Updated design doc to reflect prototype implementation and pull request number
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Commits on Mar 17, 2020

Changed laplace_diag_shift to laplace_add_diag and added more info about how constrained samples are generated as response to reviews
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Commits on Apr 20, 2020

Commits on Sep 11, 2020

Minor updates and rename of laplace approximation algorithm (design-doc #16)
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