Reducing memory footprint of the cell-pair struct

[manodeep]

Is your feature request related to a problem? Please describe.
The newly introduced struct cell-pair has a fairly large memory footprint source . Combined with ~millions of cell-pairs, the overall memory requirement can be quite significant.

Describe the solution you'd like
Only keep the cellindex1, cellindex2 and other variables that pertain to the cell-pair. All other info can be directly retrieved from lattice1[cellindex1] and lattice2[cellindex2]arrays.

Describe alternatives you've considered
N/A

Additional context
None

We might want to release this feature as a patch version 2.3.1.

[manodeep]

Closed via #186