Test geoms with maps and projections and add examples

[aphalo]

Brief description of the problem

Support for sf data for maps has been added to 'ggplot2' some time ago, but 'ggpp' has not been tested for labelling of maps.

Example user interface and/or plot output.

Add examples to documentation, and unit tests to testthat folder.

[aphalo]

Statistic stat_sf_coordinates() can be used to translate map coordinates into x and y coordinates. This needs testing and examples need to be added to the vignettes. Even if this solves the problem in many cases it leaves unresolved the problem for those cases where the elements being plotted are computed by a different statistic as statistics cannot be concatenated...