Removing high-level API classes

[ciguaran]

This is a continuation of:

#492

#501

See two solution alternatives
#657 replacing classes with objects of a single class
#658 replacing classes with functions

[reubenharry]

Thanks for looking into this! I tend to favour the more functional solution (#658)

[junpenglao]

Thank you for working on this!! This is a great suggest and i like the solutions!

So IIUC: #657 add the object to create high-level alias on top of the functional solution in #658.
While I also like #658 better, for backward compatibility we should go with #657. It should be easy to migrate/deprecate the object in the future because it is just one Python object and everything will be in the single file.

[albcab]

+1 what Junpeng said since #658 would break any script using blackjax

[junpenglao]

Fixed in #657