Add Floyd-Warshall algorithm under graphs

[daisy-amrita]

Implemented Floyd-Warshall algorithm for all-pairs shortest path. Includes example usage.

Checklist

• My code follows the contribution guidelines
• I have tested my code
• I have added example usage
• I have added documentation

[algorithms-keeper]

Closing this pull request as invalid

@daisy-amrita, this pull request is being closed as none of the checkboxes have been marked. It is important that you go through the checklist and mark the ones relevant to this pull request. Please read the Contributing guidelines.

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