Rely on numpy array to store pending ways

[agrenott]

Much more efficient memory-wise than massive lists of tuples, allowing to process much bigger dataset with a reasonable amount of memory.

Fixes #16

[codacy-production]

Coverage summary from Codacy

Merging #19 (33d68ea) into master (db38f25) - See PR on Codacy

Coverage variation	Diff coverage
✅ +0.04% (target: -1.00%)	100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (db38f25)	1442	1291	89.53%
Head commit (33d68ea)	1447 (+5)	1296 (+5)	89.57% (+0.04%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#19)	23	23	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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