Improve Hamiltonian constructors

[devmotion]

This PR proposes some changes of the Hamiltonian constructors:

• For log density functions that already support computation of the gradient as well (via logdensity_and_gradient) an AD backend is not used automatically but only if explicitly specified
• LogDensityModels are handled in the same way as if only their log density would have been provided, ie, in particular ForwardDiff is used automatically if the log density does not support computation of the gradient

[torfjelde]

Sorry, but isn't this a breaking change? Prior to this PR, you didn't have to implement the LogDensityProblems-interface; you could still just provide a callable that represented the logdensity. It seems like this is no longer possible?

[devmotion]

No, I don't think so. Already prior to this PR in AdvancedHMC 0.4, Hamiltonian(metric, lp) required that lp supported LogDensityProblems.logdensity. Arbitrary callables were only supported in the Hamiltonian(metric, lp, lp_and_grad) constructor which is not affected by this PR.

The main changes in this PR were:

• Handling model::LogDensityModel in exactly the same way as if model.logdensity would have been provided
  • previously model.logdensity required support of gradient evaluation whereas it was fine to provide a "vanilla" log density without gradient capabilities
  • previously only Hamiltonian(metric, model::LogDensityModel, ::Union{Val,Symbol}) was supported whereas Hamiltonian(metric, logdensity, ::Union{Val,Symbol,Module}) also supported Modules
• Checking if the log density function actually supports the LogDensityProblems interface with LogDensityProblems.capabilities to provide more meaningful error messages.

In principle, in a follow-up PR we could re-add support for callables which was removed in AdvancedHMC 0.4: If LogDensityProblems.capabilities(logdensity) === nothing, we could assume the provided logdensity is a function.