Nearby Markers button and marker detection

[AsquaredWsquared]

The "geo-engine"/splat of functions currently only works with data that is in a GeoJSON format. If a Marker or set of Markers is described as a GeoJSON Point/MultiPoint/whatever then it can be detected with the current geo code and we can do all of the geo stuff (distance to nearest, direction to nearest, detect Markers in a certain radius, look for a particular Marker based on description, blah).

This means that we need to choose one of these options:
Store our Markers in a GeoJSON string in the RealmDB and have the ability to export/import in the GPX format.
Store our Markers in the GPX format in the RealmDB and translate to GeoJSON.
Store our Markers in GPX format and write geo functions to handle GPX data <- not a great option as that is a lot of extra typing!

This is related to Issue #144 and how we will store Markers and Routes