First order walks and major refactoring

Summary

New features

• Implemented fast and memory efificent first-order walk modes PreCompFirstOrder and FirstOrderUnweighted (when p = q = 1, or even faster when the graph is unweighted)
• Added new gamma hyperparameter for node2vec+

Others

• Setup automated testing
• Refactor graph objects

What's Changed

• Refactor graph by @RemyLau in #43
• Refactor graph construction by @RemyLau in #50
• Test cli and remove deprecated options by @RemyLau in #51
• Refactor rwgraph by @RemyLau in #52
• First order walk by @RemyLau in #53
• Automated testing by @RemyLau in #56
• Fix windows by @RemyLau in #57
• Cleanup configs by @RemyLau in #58
• Gamma hyperparam by @RemyLau in #60

Full Changelog: v2.0.1...v2.0.2