Add support for minimum utilization calendars

[ushnisha]

Currently, workcenters support only efficiency calendars. We also need to support minimum utilization calendars. This includes:

(1) Any additional data/schema changes to represent minimum utilization calendars
(2) Planning logic to try and satisfy minimum utilization % requirements and the order in which we try to load the resources up to the minimum utilization
(3) Clearly define any assumptions in terms of the priority of a minimum utilization goal when compared to meeting demand on time, etc.