volumeOfCone implementation

[davcs86]

Closes #591

• [x] Running yarn lint does not trigger any linter errors
• [x] The test of the method you have fixed is passing
• [x] You have written a new skeleton method for someone else to work on!
• [x] You have written tests to accompany your skeleton method

[ktilcu]

@davcs86 Looks good! Thanks for the contribution and Happy Hacktobering 🎃 !

[ktilcu]

oh and if you could it would help us a bunch if you opened an issue for your new skeleton method.

[davcs86]

of course @ktilcu