Add first geohash center support

[fabienpomerol]

We already support the last geohash center, it might be usefull to also center on the first geohash received.

[amotl]

Dear Fabien,

we cherry-picked your improvements into our working branch at [1], wrote about this and further updates at [2] and just announced the outcome at [3], which you might also be interested in. Thanks a bunch for sharing your efforts.

With kind regards,
Andreas.

[1] https://github.com/hiveeyes/grafana-worldmap-panel
[2] https://community.grafana.com/t/giving-the-grafana-worldmap-panel-some-love/17210
[3] https://community.hiveeyes.org/t/grafana-worldmap-panel-0-3-0-dev-series/1824

[fabienpomerol]

@amotl i just rebase my code on the master branch, and read again your message saying you already chery-picked this feature 🏎️

Thanks for your effort. Should i close this PR ?

[amotl]

Hi @fabienpomerol. As your PR probably received some amendments after being cherry-picked, I believe it is reasonable to close this.

@daniellee promised to review the code which you can pull in the meanwhile from hiveeyes/develop and which should include the feature you added the other day and some more others you might find handy.

Thanks again!