added new funciton inv to stan
added new function inv_sqrt
added new function inv_square
Merge branch 'develop' of https://github.com/stan-dev/stan into feature/fix-grammar-assoc-expected-op
made precedence of >> and > explicit in grammar rules
Added log1m_exp: math, gradients, tests and doc
Added log_diff_exp: math, gradients, tests and doc
Improvement to ordered_logistic distribution: now it uses log1m_exp(.) instead of log1m(exp(.)).
added include of <string> in ast.hpp
Merge branch 'develop' of https://github.com/stan-dev/stan into feature/fix-grammar-assoc-expected-op
Merge branch 'develop' into feature/adding_inv_functions
added inv, inv_sqrt, inv_square to doc
Merge branch 'develop' of github.com:stan-dev/stan into develop
Decrease improper posterior threshold for stepsize to account for floating point precision noise
fixed inv, inv_sqrt, inv_square functions and added boundary tests
updating make instructions for generating src/stan/models/model_header.hpp.d
refined boundary tests for inv, inv_sqrt, inv_square
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Catch init_stepsize when initializating sampler, add tests
Tweak output to mirror old notation, update tests