Allow for exponents and powers in rate expressions

[stevencarlislewalker]

For example here is a model that requires them:

https://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1005453

I'm told exponents are important for something called 'phenomenological heterogeneity', which I understand very loosely.

• [x] Develop proposal for extending the data structure for rate matrices that gets passed to TMB to allow for exponents and powers
• [x] Update C++ code to utilize this data structure
• [x] Update R code to allow for this data structure
• [x] Merge dev branch (not pushed) back into tmb-condense in a manner that is coordinated with @guanwg 's work on additional loss functions