Model basic infrastructure

[perrydv]

This establishes:

• predefined nClass definitions for modeBase_nClass and nodeFxnBase_nClass
• C++ code in the predefined files for CRTP layers for each
• functions for making a model nClass from a new nimbleModel model object
• predefined nClass definitions for calculation instructions:
  • calcInstrVec_nClass to hold a vector of calculation instructions
  • calcInstr_nClass to hold one calculation instruction with a vector of node instructions
  • nodeInstr_nClass to hold one node instruction
• A function to put together a calcInstrVec_nClass and its components from an R list of inputs.
• Various updates to core code to support the above.