Better support for target_observation_weights

[topipa]

This PR does a few improvements to make the package more generalizable.

• Move all stan related packages to "Enhances". Currently rstan is in "Suggests", but I don't see why rstan should be treated differently
• Move the basic functions related to brmsfit objects to function-specific folders just like other stan interfaces. Only functions solely used for brmsfit objects stay in a separate file
• Adds better support for target_observation_weights to make it easier to do loo calculations.
• Adds tests for using target_observation_weights with different stan objects.

TODO:

• Add tests also for cmdstan objects
• Implement cmdstanfit and stanfit methods so that they also update & return the model object.
• Make all tests work again
• Come up with a better way to easily handle expectations computed using constrained parameters with brms, rstan & cmdstanfit without user having to do anything special.