Register GPX.jl to General registry

[s-celles]

Following #4 (comment)
GPX.jl should be registered

[Wikunia]

Hi @scls19fr Would love to use this package for a new project of mine: https://github.com/Wikunia/EverySingleStreet.jl
For this it needs to be registered. Would it be okay for you to register your package? Thanks in advance and let me know if you have any concerns or anything I can help you with.

[s-celles]

I'm quite busy and I'm not using this anymore.
Feel free to fork this repository and register it.