deprecate int_step

[bob-carpenter]

Summary:

int_step(x) and step(x) return different values for 0.

The version of step(x) matches the BUGS definition and the standard definition of the Heaviside step function. Therefore, it should not be changed.

Changing int_step()'s behavior would be confusing, so instead it should be deprecated and replaced with a suggestion to replace existing uses of int_step(n) with the boolean expression (n >= 0), which evaluates to 1 if n >= 0 and 0 otherwise.

Current Version:

v2.17.0

[roualdes]

I just submit 2 pull requests, one in #2501, which matches tests across int_step.stan and step.stan and one in stan-dev/math#805, in which the step function definitions were changed and tests were updated. Review when convenient, please. Thanks.

[bob-carpenter]

Thanks. I'll get to these ASAP.