Add demand based frequency to the exact_times field of frequencies.txt

### Describe the problem

During the Improving Real-Time Data Quality workshop at the MobilityData Summit people at our table realized the similarities in service between MBTA rail replacement shuttles and the micro-buses of Cairo. They both wait at the first stop until they are full and then start on their route.

### Use cases

Cases where there is no fixed schedule or frequency that is being followed.

- Subway and rail shuttle bus replacements where buses depart when they are full, often with a wave of vehicles after train has arrived from the part of the line that is still open.
- Popular transport where privately owned buses depart on their own time, usually when they have enough passengers.
- Vehicles being run for a special event where there may be a wave of trips departing when the event ends.

### Proposed solution

A new option for the `exact_times` enum in the frequencies.txt file which would indicate trips will depart based on the number of passengers at the first stop and other factors that may not be predictable and the headway is only average to indicate the overall level of service.

### Additional information

I'm not in a position to be the advocate for this change. I don't even use frequencies.txt in TransSee right now. It's just an idea that I thought should be shared.