Allow users to use their own macros and `~` inside a DPPL model with $

[mohamed82008]

It would be nice to allow users to use $ around some expression (e.g. a macro call) to avoid any fiddling with it on the DPPL side. This for example allows users to make use of their own DSL inside of a @model block. This can include their own ~ and .~ and what not that we use in DPPL but they can have different semantics inside their block of code.

[mohamed82008]

I believe the Pumas team (@simonbyrne ?) wanted this feature at some point in the past. This would also make it easier to integrate Soss and Gen models into Turing in the future.

CC: @ChrisRackauckas

[ChrisRackauckas]

Yeah DiffEqBayes and Pumas. Pumas now just targets AdvancedHMC directly, and DiffEqBayes is a bit more experimental and targets the function that the @model macro would generate directly.