Initialize Lattices from networkx.Graphs

[Cryoris]

Summary

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Edit: This PR is now split into #471 and #472.

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Fixes #465: Support the initialization of lattices from networkx graphs.
Fixes #466: Allow only numerical values for edge weights.

Details and comments

Now we can for instance construct lattices as

import networkx as nx
from qiskit_nature.problems.second_quantization.lattice import Lattice
# 3-regular random graph on 6 nodes
graph = nx.generators.random_graphs.random_regular_graph(3, n=6)
lattice = Lattice(graph)

[coveralls]

Pull Request Test Coverage Report for Build 1593395376

• 14 of 16 (87.5%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 85.38%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit_nature/problems/second_quantization/lattice/lattices/lattice.py	14	16	87.5%

Totals
Change from base Build 1593381601:	0.02%
Covered Lines:	10115
Relevant Lines:	11847

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💛 - Coveralls

[Cryoris]

Closing in favor of splitting this PR in two: the edge type check #471 and the networkx support #472.