Adding 'asset_group' capability to the scheduler

- Eliminated the lower bound constraint for Constraint 3, which allowed multiple assets to be assigned to the same task
- - It is now an equality constraint, where each asset (column in the task-asset matrix) corresponds to an 'Asset Group', which is a combination of assets that is used to perform each task
- - This prevents multiple asset groups from being used by the same task, but this means that many asset groups need to be defined to cover all asset combination possibilities
- - - Another script is in the works to generate the task-asset matrix based on asset capabilities and task requirements, but for now, we can just look at the example at the bottom of the scheduler.py script
- We initialize new asset groups, which generate dictionaries in '_initialize_asset_groups', which are now used by Constraint 4 to ensure that the same asset used in different asset groups is not used by multiple tasks in the same time period
- A small print help

- Initial commit for the scheduler_tutorial.ipynb to detail how the scheduler works

Author: shousner