- Cube Grouping
- New sub-cube hash table

Enables FXCH to handle Distance-1 cubes (D1C) and Single Cube Containment (SCC) as by product of extraction.

D1C: Whenever they appear a constant divisor (x! + x)  will be created and handle as any other divisor.
SCC: Will be taken care of as soon as they appear.

With this change, 'empty' sub-cubes (sub-cubes with an ID equal to zero) are inserted  in the 0th bin of the hash table.

Removes old partial implementation of this algorithm from the "pla" module.