manual information for macros

[paciorek]

WIP: This PR is for us to discuss content related to macros for the nimble manual.

I've taken a stab at giving some high level information and brief examples (copied from Ken's nimbleMacros vignette) into the Writing Models and User-defined chapters.

Related notes:

• I think we can basically leave all of what Ken has in the nimbleMacros vignette there.
• Furthermore, Ken has a full set of roxygen in both nimble and nimbleMacros for user-facing macro-related functions.

@kenkellner @perrydv @danielturek Let me know of any comments/suggestions.

[kenkellner]

I removed the reference to LINPRED_PRIORS.

I also made some changes to the code.

• It's not clear to me if these code blocks are meant to be executed since most of the other chunks in the file are eval=FALSE (but these ones are not). If they are meant to be run I think we need to load nimbleMacros.
• If splitting constants and data, need to specify the index range of weight (it needs to be initially in the constants in order for LM to auto detect that index)
• I tried running nimbleMCMC on both models. First one worked OK. In the second one I found that if the SD has dhalfflat prior there are MCMC issues. Maybe need to set inits? Instead of doing that I just changed to a uniform prior which works fine, but happy with whatever you think is best here.