infrastructure for nimble models

[perrydv]

Add a variety of infrastructure to facilitate new nimble model implementation using nClass and nFunction.

• Add isCompiled() method to the R6 classes for uncompiled and compiled nClass.
• Allow user to provide an (R) initialize; old steps there move to initializeCpp.
• Get nimbleModel into CI testing setup using a private token until we move that to a public repo which there should be no reason not to do.
• Update tests.