Provide per proposal defaults for target acceptance rate and initial scale

[matt-graham]

Provides defaults for target_accept_prob and initial_scale parameter to scale adapters which are proposal (and for initial_scale dimension) dependent using 'optimal' values from Roberts and Rosenthal (2001)

• Random walk proposal: target_accept_prob = 0.234 and initial_scale = 2.38 / (dimension)^(1/2)
• Langevin proposal: target_accept_prob = 0.574 and initial_scale = 2.38 / (dimension)^(1/3)

and recommend values from Livingstone and Zanella (2022) for Barker proposal of target_accept_prob = 0.4 and initial_scale = 2.38 / (dimension)^(1/3).

[codecov[bot]]

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