Added
- Remove factors from requirements if already implied by other requirement
list.
- Added tiling method `is_empty_cell` and `is_monotone_cell`
Changed
- The `cell_basis` method of the tilings has an 1 obstruction for empty cell.
The basis of a cell that is outside of the tiling is no longer defined.
- The requirement list in `cell_basis` method now finds intersections of
requirement lists
- New `add_list_requirement` method to `Tiling`.
Fixed
- Infinite recursion issue in get_genf.
- Close mongo when finished.