added one branch of vvv for log_mix, with tests, rest to do
Merging
Merge branch 'feature/issue-970-mix-fun' of https://github.com/stan-dev/stan into feature/issue-970-mix-fun
b gt a version of log_mix.hpp var added along with tests
Added vv versions of log_mix, along with tests. Added precomp_vv_vari
variable.
Added precomp_v versions of tests and function for log_mix
Barebones tests for precomp_v_vari, precomp_vv_vari, and precomp_vvv_vari
Cleaned up tests for precomp_v:vvv_vari functions
Added fwd log_mix and began writing testing fwd log_mix
2nd derivative tests for fwd log_mix
Log_mix tests through third derivatives
Tweaks to log_mix_test.cpp
Redefinition of log_mix, finished up log_mix_test fwd
Updated function signatures, the parser test and added test model
Merge branch 'develop' into feature/issue-970-mix-fun
Changed name of is_same struct to mult_wrapper and using arrays rather than vectors.
Wrote var version of log_mix using OperandsAndPartials
Minor change to lambda2 > lambda1 deriv calcs
Making function names clearer
Adding doc and simplifying function names
Further function simplification