Added BaseGeoLayer base class

[gentlegiantJGC]

Adds BaseGeoLayer that FileMarker extends and in the future a GeoJSON class could be added as well. BaseGeoLayer outlines shared attributes and methods that each subclass must define.

Modifies many functions that were previously only aware of the FileMarker class to use BaseGeoLayer. FileMarker only code is moved behind an instance check. Other classes will currently throw an exception. Perhaps in a future pull request that adds GeoJSON support, the class specific code should moved into a method of the class.

Resolves #109