Arrays of draws of arbitrary dimension

[paul-buerkner]

In brms, I need draws arrays in various places that hold mulitple dimensions of draws and I feel we should support them natively in posterior. Such arrays should have the following properties:

• Iterations/draws are always the first dimension
• All other dimensions can be freely named and otherwise receive default names
• no chain information is stored unless manually done be the user (in contrast to the current draws_array class, which will remain in the current form)
• subsetting should not lead to dropping dimensions
• methods subset, summarise_draws, etc. work canonically on these objects

We should come up with a good name for this class. The question is mostly which of the two array classes should be called draws_array and which should receive another name.

[paul-buerkner]

This will be implemented as part of #8. @mjskay, I would still like the principles in the OP to be taken into account in #8 but we can discuss this once you start working on #8 again.

[paul-buerkner]

Implemented via #106