Provide capability to use custom map layer

[JaWeilBaum]

Research and test if there is a possibility to use a local map layer and there is code needed to make this available to everyone.

[JaWeilBaum]

This can be done via L.layer as the basic layer.

You can also use local tiles which need to have a correct path to your working directory. (Works under MacOS, Windows not yet tested)