Reenable PriorContext for Optimization

[alyst]

This PR reenables the support for PriorContext in optimization, which was disabled by #2022.
While it is not directly supported by the current optim_problem() interface (only MLE/MAP are supported, although adding MaxPrior is possible), I think it is useful to support PriorContext for Turing.OptimLogDensity().
In my code I use it for multi-objective Pareto optimization (prior + llh).

[devmotion]

This PR reenables the support for PriorContext in optimization, which was disabled by #2022.

AFAICT support for PriorContext has never existed and has never been tested. The check in the PR was only introduced to communicate this more clearly and provide a better error message.

[alyst]

AFAICT support for PriorContext has never existed and has never been tested.

I see. It was working before though. Do you think it makes sense to support it for some of the user use cases?

[torfjelde]

IMO if we 're allowing both DefaultContext and LikelihoodContext, I feel like allowing PriorContext also seems reasonable (i.e. all the leaf-contexts).

[mhauru]

I had one minor style change to suggest.

I also got some extra peace of mind by, in the test, checking that

            true_prior_logpdf = -Distributions.logpdf(Uniform(0, 2), a[1])
            @test Turing.OptimLogDensity(m1, prictx)(a) ≈ true_prior_logpdf

Could incorporate a more explicit check like that.

Both of these are optional, I'd be okay with the code as is. I'll leave approving for @torfjelde since there might be context (pun unintended) to this that I lack.

[alyst]

@mhauru I have incorporated your suggestions, thank you!

[torfjelde]

Would be happy to include this:) But we need to fix the merge conflicts 😕

[coveralls]

Pull Request Test Coverage Report for Build 9384233247

Details

• 0 of 5 (0.0%) changed or added relevant lines in 1 file are covered.
• 89 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 0.0%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/optimisation/Optimisation.jl	0	5	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/optimisation/Optimisation.jl	89	0.0%

Totals
Change from base Build 9374257750:	0.0%
Covered Lines:	0
Relevant Lines:	1503

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💛 - Coveralls

[mhauru]

Looks good to me

[mhauru]

Thanks @alyst!

[torfjelde]

Lovely work @alyst :)