Add ability to specify number of concurrent inputs/outputs to scheduler

The user should be able to e.g. specify that the schedule produced by a ListScheduler should have at most two inputs at the same time step in the schedule.

added Scheduling label

assigned to @simbj106

This could be given in the max_resources dict, e.g. {Input.type_name(): 1, Output.type_name(): 2}

The user should also be able to specify whether the output should wrap around cyclically.