Tuple datatype io

[ahartikainen]

Describe the bug

Stan 2.33 brings us Tuple datatype. I think we need to handle this with a custom numpy dtype depending on the tuple type.

How can we access tuple type and pack incoming data correctly? And how should user give out the data for inits / data.

Custom numpy dtypes are easy to create and also support access to individual components, but we probably should either have small example how to work with them or a functionality to unpack them.