Add is_present_expr() method for IntervalVar

[espeon011]

I am modeling scheduling problems by making extensive use of optional interval variables.

Currently, the only explicit way to know whether an optional interval variable is active or not, is to separately store the arguments passed to new_optional_interval_var(..., is_present, ...).
This means we must maintain both an interval variable and a boolean variable.

I suggest adding an is_present_expr() method to the API, similar to start_expr(), size_expr(), and end_expr().
This would allow users to build more concise models without having to maintain two decision variables.

[lperron]

implemented on the main branch.