New User-Facing Features
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* introduction of = for assignment; deprecating <- (#1819)
* _lpdf, _lpmf, _lcdf, _lccdf and vertical bar notation
for probability mass, density, and cumulative distribution
functions; update user-defined function syntax;
deprecate older forms (#1811)
* target += lp to replace increment_log_prob(lp); target() to rpelace
log_prob() function; remove lp__ variable access (#1800)
* conditional (ternary) operator (#1125)
* stiff diff eq solver CVODES from Sundials
* add control parameters (tolerance, max iteraitons) to ODE solvers
* rename ODE solvers based on algorithm, integrate_ode_rk45
for existing non-stiff Runge-Kutta solver and integrate_ode_bdf
for the stiff backward differentiation form; deprecate
the unmarked integrate_ode function (#1886)
* limiting diff eq iterations in solvers (Boost/CVODES)
* unit_vector as parameter (#1713) [it never worked in the past]
* rename multiply_log and log_binomial_coefficient
to lmultiply and lchoose (also part of #1811)
* incomplete beta function as inc_beta (#1540)
Daniel