added hectares to convertArea

[DanielJDufour]

Hello. This pull request adds hectares as a unit to convertArea inside of turf-helpers.

• Use a meaningful title for the pull request. Include the name of the package modified.
• Have read How To Contribute.
• Run npm test at the sub modules where changes have occurred.
• Run npm run lint to ensure code style at the turf module level.

This is my first submission to Turf. I apologize in advance for anything I might have missed. Happy to provide more details. Thank you for your consideration.

References:

• https://en.wikipedia.org/wiki/Hectare
• googling convert 1 meter to hectares which displays

[mfedderly]

Thanks!