Add timezone lookup from lat/lon

[alorenzo175]

using either https://github.com/darkskyapp/tz-lookup-oss or https://github.com/jannikmi/timezonefinder baked into the API

[wholmgren]

For what it's worth, a few people have recommended timezonfinder in pvlib issues/prs. I also found https://github.com/evansiroky/node-geo-tz by clicking through recommendations on a few other neglected packages. Seems like there's a lot of tz lookup abandonware.