Updates to graph class (px2ang and max_neighbors are added)

[saimani5]

--Added px2ang argument for the graph class. For finding the neighbors, coordinates in Angstroms are used, and for making nx_graph, coordinates in pixel units are used so that the plots make sense.

--Added max_neighbors as a kwargs argument to the find_neighbors method in the graph class. When set to -1 (default value), the method works like the one in the previous version. When set to any other values, the number of neighbors for each node is cut off at the 'max_neighbors.' Additional care is also taken to avoid unidirectional edges. An edge is formed only when the nodes are connected to each other after satisfying the 'max_neighbors' argument from both sides.

[codecov-commenter]

Codecov Report

Merging #66 (bc503f3) into master (9a528f2) will decrease coverage by 0.21%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
- Coverage   74.86%   74.64%   -0.22%     
==========================================
  Files          63       63              
  Lines        6474     6493      +19     
==========================================
  Hits         4847     4847              
- Misses       1627     1646      +19     

Flag	Coverage Δ
unittests	74.64% <0.00%> (-0.22%)	:arrow_down:

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Impacted Files	Coverage Δ
atomai/utils/graphx.py	9.68% <0.00%> (-0.78%)	:arrow_down:

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[ziatdinovmax]

@saimani5 Thank you for these updates! Can you please also add a few simple unit tests?