Support nimble2 with various features

[perrydv]

This PR brings in features for supporting a new version of nimble with nimbleModel. These include:

• renovated system for managing compile-time information on nClass inheritance in the project_env.
• fixed determination of nested types, multiple levels in (A$B$C) when there were TBD types to be resolved.
• fleshed out get_interface_ptr feature of the generic interface in C++.
• add C++ classes RuntimeFlatView and RuntimeFlatViewGroup to support nimble's values() keyword.
• other bug fixes and details

@paciorek I don't think this needs a rebase. OK to merge and then you can rebase your TBB branch (again)?

[paciorek]

@perrydv I think this has broken nimbleModel master branch. Not sure if I need to switch to the branch of nimbleModel with the nimble2 changes?

code <- quote({
        tau ~ dunif(0, 100)
        mu ~ dnorm(0,1)
        for(i in 1:5) {
            y[i] ~ dnorm(mu, var = tau)
        }
    })

    inits <- list(tau = 25, mu = 0)
    data <- list(y = rnorm(5))

    ## Manual workflow
    mclass <- nimbleModel(code, inits = inits, data = data, returnClass = TRUE)
    m <- mclass$new()
    cmclass <- nCompile(mclass)
    cm <- cmclass$new()

Error in NCint$process_inherit(inheritInfo = inheritInfo, symbolTable = symbolTable,  : 
  unused arguments (inheritInfo = inheritInfo, symbolTable = symbolTable, project_env = project_env)

[perrydv]

@paciorek have you tried rebuilding nimbleModel with the updated nCompiler installed? There is the issue that calls to nClass in nimbleModel will bake in nClass features from nCompiler, so if nCompiler updates, nimbleModel can break for that reason. Eventually we may want many or all nClasses created in a package like nimbleModel to be created in .onLoad.

[paciorek]

@perrydv thanks; that resolved the problem. You've probably told me about this before but I haven't absorbed it...