kendalfoster

add 5-parameter diffusion drift; fixes #2584

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## Summary This pull request implements the 5-parameter variant of the DDM by adding two new functions: `ddm_lpdf()` and `ddm_lcdf()`. This pull request addresses [issue #2584: Adding 5-Parameter Drift Diffusion...

Adding 5-Parameter Drift Diffusion Model (DDM) PDF and CDF to Stan Math

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## Description Ratcliff's Diffusion Decision Model (DDM) is a popular model for jointly modelling binary responses and their associated response times. The DDM exists in several parameterizations, and currently Stan...