Implemented greedyGraphColoring in java

This PR implements the code for greedyGraphColoring in Java referred to in issue #234

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[x] My code hasn't been implemented or claimed by someone else and I have checked the present issues and PRs for the same
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[x] If my code is the first implementation of greedyGraphColoring in java, I have added the link to this directory in README.md
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deutranium / Algorithms