Implement logpdf and loglikelihood for ProductDistribution

[itsdfish]

See discussion here.

[itsdfish]

@PTWaade, I added logpdf and rand methods for SSM2D that work for product_distribution. It works for SSM1D out of the box, but there were some complications for ContinuousMultivariateSSM. I will do some more investigation later.

I'll be releasing a new version soon with the new methods.

[itsdfish]

@JuliaRegistrator register()

[JuliaRegistrator]

Registration pull request created: JuliaRegistries/General/134564

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## Breaking changes

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[PTWaade]

@itsdfish wonderful news - I will try it out as soon as I get the chance :)

Thank you, excited to use this more! I don't have any immediate use-cases with ContinuousMultivariateSSM models, but it would be great to be able to do product distributions with those as well.

Many warm thoughts from here