Filters for Pair Indexers

[sakibmatin]

PyTorch module to filter various pair_lists (pair_dist, pair_first, ...) by distance in pair_first. hippynn/hippynn/layers/pairs/filters.py

Graph Nodes OpenFilter and PeriodicFilter. Structure is similar to existing OpenPairIndexer and PeriodicPairIndexer hippynn/hippynn/graphs/nodes/pairs.py

ASE graph construction automatically uses Filters whenever multiple PairIndexers are present in the model.
hippynn/hippynn/interfaces/ase_interface/calculator.py

TODO

• Doc Strings.
• Remove GraphViz code in ase_interface/calculator.py
• Unit Tests

[sakibmatin]

Can be merged in. The Unit test for ASE graph construction may be overkill.